## xfsdump Internals

### Table Of Contents

- Linux Caveats
- What's in a dump
- Dump Format
  - Media Files
  - Inode Map
  - Directories
  - Non-directory files
- Format on Tape
- Run Time Structure
- xfsdump
  - Control Flow of xfsdump
    - The main function of xfsdump
      - drive_init1
      - content_init
    - Dumping to Tape
      - content_stream_dump
      - dump_file_reg
  - Splitting a Regular File
    - Splitting a dump over multiple streams
- xfsrestore
  - Control Flow of xfsrestore
  - Persistent Inventory and State File
  - Restore's directory entry tree
  - Cumulative Restore
    - Cumulative Restore Tree Postprocessing
  - Partial Registry
- Drive Strategies
  - Drive Scsitape
    - Reading
  - Librmt
  - Drive Minrmt
  - Drive Simple
- Online Inventory
- Questions and Answers
  - How is -a and -z handled by xfsdump ?
  - How does it compute estimated dump size ?
  - Is the dump size message accurate ?
- Outstanding Questions

### Linux Caveats

These notes are written for xfsdump and xfsrestore in IRIX. Therefore, it refers to some features that aren't supported in Linux. For example, the references to multiple streams/threads/drives do not pertain to xfsdump/xfsrestore in Linux. Also, the DMF support in xfsdump is not yet useful for Linux.

### What's in a dump

Xfsdump is used to dump out an XFS filesystem to a file, tape or stdout. The dump includes all the filesystem objects of:

- directories (S_IFDIR)
- regular files (S_IFREG)
- sockets (S_IFSOCK)
- symlinks (S_IFLNK)
- character special files (S_IFCHR)
- block special files (S_IFBLK)
- named pipes (S_FIFO)

It does not dump files from /var/xfsdump which is where the xfsdump inventory is located. Other data which is stored:

- file attributes (stored in stat data) of owner, group, permissions, and date stamps
- any extended attributes associated with these file objects
- extent information is stored allowing holes to be reconstructed on restoral
- actual file data of the extents

### Dump Format

The dump format is the layout of the data for storage in a dump. This is mostly done at an abstraction above the media dump format (tape or data file). The tape format, for example, will have extra header records. The tape format will be done in multiple media files, whereas the data file format will use 1 media file.

#### Media Files

Media files are probably used to provide a way of recovering more data in xfsrestore(1) should there be some media error. They provide a self-contained unit for restoration. If the dump media is a disk file (drive_simple.c) then I believe that only one media-file is used. Whereas on tape media, multiple media files are used depending upon the size of the media file. The size of the media file is set depending on the drive type (in IRIX): QIC: 50Mb; DAT: 512Mb; Exabyte: 2Gb; DLT: 4Gb; others: 256Mb. This value (media file size) is now able to be changed by the "-d" option. . Also, on tape, the dump is finished by an inventory media file followed by a terminating null media file.

A global header is placed at the start of each media file.

    global_hdr_t (4K bytes)
    magic# = "xFSdump0"
    version#
    checksum
    time of dump
    ip address
    dump id
    hostname
    dump label
    pad to 1K bytes
    drive_hdr_t (3K bytes)
        drive count
        drive index
        strategy id = on-file, on-tape, on-rmt-tape
        pad to 512 bytes

        specific (512 bytes)
            tape:
    	    rec_hdr
    	    magic# - tape magic = 0x13579bdf02468acell
    	    version#
    	    block size
    	    record size
    	    drive capabilities
    	    record's byte offset in media file
    	    byte offset of rirst mark set
    	    size (bytes) of record containing user data
    	    checksum (if -C used)
    	    ischecksum (= 1 if -C used)
    	    dump uuid
    	    pad to 512 bytes

        upper: (2K bytes)
    	media_hdr_t
    	media-label
    	previous media-label
    	media-id
    	previous media-id
    	5 media indexes - (indices of object/file within stream/media-object)
    	strategy id = on-file, on-tape, on-rmt-tape
    	strategy specific data:
    	  field to denote if media file is a terminator (old fmt)
    	upper: (to 2K)

Note that the strategy id is checked on restore so that the dump strategy and the strategy used by restore are the same with the exception that drive_scsitape matches with drive_minrmt. This strategy check has caused problems with customers in the past. In particular, if one sends xfsdump's stdout to a tape (i.e. xfsdump -L test -M test - / >/dev/tape) then one can not restore this tape using xfsrestore by specifying the tape with the -f option. There was also a problem for a time where if one used a drive with the TS tape driver, xfsdump wouldn't recognise this driver and would select the drive_simple strategy.

#### Inode Map

#### Directories

#### Non-directory files

Regular files, as can be seen from above, have a list of extents followed by the file's extended attributes. If the file is large and/or the dump is to multiple streams, then the file can be dumped in multiple records or extent groups. (See Splitting a Regular File).

### Format on Tape

At the beginning of each tape record is a header. However, for the first record of a media file, the record header is buried inside the global header at byte offset 1536 (1K + 512), as is shown in the global header diagram. Reproduced again:

    rec_hdr
    magic# - tape magic = 0x13579bdf02468acell
    version#
    block-size
    record-size
    drive capabilities
    record's byte offset in media file
    byte offset of rirst mark set
    size (bytes) of record containing user data
    checksum (if -C used)
    ischecksum (= 1 if -C used)
    dump uuid
    pad to 512 bytes

I can not see where the block-size ("tape_blksz") is ever used ! The record-size ("tape_recsz") is used as the byte count to do the actual write and read system calls.

There is another layer of s/ware for the actual data on the tape. Although, one may write out an inode-map or directory entries, one doesn't just give these record buffers straight to the write system call to write out. Instead, these data objects are written to buffers (akin to <stdio>). Another thread reads from these buffers (unless its running single-threaded) and writes them to tape. Specifically, inside a loop, one calls do_get_write_buf, copies over the data one wants stored and then calls do_write_buf, until the entire data buffer has been copied over.

### Run Time Structure

This section reviews the run time structure and failure handling in dump/restore (see IRIX PV 784355). The diagram below gives a schematic of the runtime structure of a dump/restore session to multiple drives.

    1.           main process	main.c
    	       /  |   \
    	      /   |    \
    2.	stream  stream  stream	dump/content.c restore/content.c
           manager  manager manager
    	   |      |      |
    3.	 drive  drive   drive	common/drive.[hc]
    	object  object  object
    	   |      |      |
    4.	   O      O      O	ring buffers common/ring.[ch]
               |      |      |
    5.	worker  worker  worker	ring_create(... ring_worker_entry ...)
    	thread  thread  thread
    	   |      |      |
    6.	 drive  drive   drive	physical drives
    	device  device  device

Each stream is broken into two threads of control: a stream manager; and a drive manager. The drive manager provides an abstraction of the tape device that allows multiple classes of device to be handled (including normal files). The stream manager implements the actual dump or restore functionality. The main process and stream managers interact with the drive managers through a set of device ops (e.g.: do_write, do_set_mark, ... etc).

The process hierachy is shown above. main() first initialises the drive managers with calls to the drive_init functions. In addition to choosing and assigning drive strategies and ops for each drive object, the drive managers intialise a ring buffer and (for devices other than simple UNIX files) sproc off a worker thread that that handles IO to the tape device. This initialisation happens in the drive_manager code and is not directly visible from main().

main() takes direct responsibility for initialising the stream managers, calling the child management facility to perform the sprocs. Each child begins execution in childmain(), runs either content_stream_dump or content_stream_restore and exits with the return code from these functions.

Both the stream manager processes and the drive manager workers set their signal disposition to ignore HUP, INT, QUIT, PIPE, ALRM, CLD (and for the stream manager TERM as well).

The drive manager worker processes are much simpler, and are initialised with a call to ring_create, and begin execution in ring_worker_func. The ring structure must also be initialised with two ops that are called by the spawned thread: a ring read op, and a write op. The stream manager communicates with the tape manager across this ring structure using Ring_put's and Ring_get's.

The worker thread sits in a loop processing messages that come across the ring buffer. It ignores signals and does not terminate until it receives a RING_OP_DIE message. It then exits 0.

The main process sleeps waiting for any of its children to die (ie. waiting for a SIGCLD). All children that it cares about (stream managers and ring buffer workers) are registered through the child manager abstraction. When a child dies wait status and other info is stored with its entry in the child manager. main() ignores the deaths of children (and grandchildren) that are not registered through the child manager. The return status of these subprocesses is checked and in the case of an error is used to determine the overall exit code.

We do not expect worker threads to ever die unexpectedly: they ignore most signals and only exit when they receive a RING_OP_DIE at which point they drop out of the message processing loop and always signal success.

Thus the only child processes that can affect the return status of dump or restore are the stream managers, and these processes take their exit status from the values returned by content_stream_dump and content_stream_restore.

### xfsdump

#### Control Flow of xfsdump

Below is a higher level summary of the control flow. Further details are given later.

- initialize the drive strategy for a tape, file, minimal remote tape
- create the global header

content_init (xfsdump version)

Do up to 5 phases, which create and prune the inode map, calculate an estimate of the file data size and using that create inode-ranges for multi-stream dumps if pertinent.

- phase 1: create a subtree list based on the -s subtree spec.
- phase 2: create the inode map
  The inode map stores the type of the inode: directory or non-directory, and a state value to say whether it has changed or not. The inode map is built by processing each inode (using bulkstat) and in order to work out if it should be marked as changed, by comparing its date stamp with the date of the base or interrupted dump. We also update the size for non-dir regular files (bs_blocks * bs_blksize)
- phase 3: prune the unneeded subtrees due to the set of unchanged directories or the subtrees specified in -s (phase 1). This works by marking higher level directories as unchanged (MAP_DIR_NOCHNG) in the inode map.
- phase 4: estimate non-dir (file) size if pruning was done since phase 2. It calculates this by processing each inode (using bulkstat) and looking up the inode map to see if it is a changed non-dir (file). If it is then it uses (bs_blocks * bs_blksize) as in phase 2.
- phase 5: if we have multiple streams, then it splits up the dump to try to give each stream a set of inodes which has an equal amount of file data. See the section on "Splitting a dump over multiple streams" below.

- if 1 stream, then we call content_stream_dump and if multi stream, then we create children sprocs which call content_stream_dump.

content_stream_dump

- write global header
- loop dumping media files
  - dump the changed/needed directories by processing all inodes from bulkstat
    - dump the filehdr based on the bulkstat structure
    - dump the directory entries (using getdents())
    - dump a null dirent terminator
    - dump extended attributes on directory if it has them
  - dump the changed/needed files by processing all inodes from bulkstat (check the multistream range to see if it should be dumped by this particular stream)
    - dump the filehdr
    - dump the extents (called extent groups - max at 16Mb)
      - align to page boundary by dumping EXTENTHDR_TYPE_ALIGN records
      - dump data as EXTENTHDR_TYPE_DATA records
    - dump a null terminator, EXTENTHDR_TYPE_LAST
  - if not EOM then write null file header
  - end the media file
  - update online inventory
- if multiple-media dump (i.e. tape dump and not file dump) then
  - dump the session inventory to a media file
  - dump the terminator to a media file

##### The main function of xfsdump

    * drive_init1 - initialize drive manager for each stream
      - go thru cmd options looking for -f device
      - each device requires a drive-manager and hence an sproc
        (sproc = IRIX lightweight process)
      - if supposed to run single threaded then can only
        support one device

      - ?? each drive but drive-0 can complete file from other stream
      - allocate drive structures for each one -f d1,d2,d3
      - if "-" specified for std out then only one drive allowed

      - for each drive it tries to pick best strategy manager
        - there are 3 strategies
          1) simple - for dump on file
          2) scsitape - for dump on tape
          3) minrmt - minimal protocol for remote tape (non-SGI)
        - for given drive it is scored by each strategy given
          the drive record which basically has device name,
          and args
        - set drive's strategy to the best one and
          set its strategy's mark separation and media file size
        - instantiate the strategy
          - set flags given the args
          - for drive_scsitape/ds_instantiate
    	  - if single-threaded then allocate a buffer of
    	    STAPE_MAX_RECSZ page aligned
    	  - otherwise, create a ring buffer
          - note if remote tape (has ":" in name)
          - set capabilities of BSF, FSF, etc.

    * create global header
      - store magic#, version, date, hostid, uuid, hostname
      - process args for session-id, dump-label, ...

    * if have sprocs, then install signal handlers and hold the
      signals (don't deliver but keep 'em pending)

    * content_init

      * inomap_build() - stores stream start-points and builds inode map

      - phase1: parsing subtree selections (specified by -s options)
        INPUT:
    	- sub directory entries (from -s)
        FLOW:
    	- go thru each subtree and
    	  call diriter(callback=subtreelist_parse_cb)
    	  - diriter on subtreelist_parse_cb
    	    - open_by_handle() on dir handle
    	    - getdents()
    	    - go thru each entry
    		- bulkstat for given entry inode
    		- gets stat buf for callback - use inode# and mode (type)
    		- call callback (subtreelist_parse_cb())
    	  * subtreelist_parse_cb
    	    - ensure arg subpath matches dir.entry subpath
    	    - if so then add to subtreelist
    	    - recurse thru rest of subpaths (i.e. each dir in path)
        OUTPUT:
    	- linked list of inogrp_t = pagesize of inode nums
    	- list of inodes corresponding to subtree path names

      - premptchk: progress report, return if got a signal

      - phase2: creating inode map (initial dump list)
        INPUT:
          - bulkstat records on all the inodes in the file system
        FLOW:
          - bigstat_init on cb_add()
    	  - loops doing bulkstats (using syssgi() or ioctl())
    	    until system call returns non-zero value
    	  - each bulkstat returns a buffer of struct xfs_bstat records
    	    (buffer of size bulkreq.ocount)
    	  - loop thru each struct xfs_bstat record for an inode
    	    calling cb_add()
    	  * cb_add
    	    - looks at latest mtime|ctime and
    	      if inode is resumed:
    		 compares with cb_resumetime for change
    	      if have cb_last:
    		 compares with cb_lasttime for change
    	    - add inode to map (map_add) and note if has changed or not
    	    - call with state of either
    		changed - MAP_DIR_CHANGE, MAP_NDR_CHANGE
    		not changed - MAP_DIR_SUPPRT or MAP_NDR_NOCHNG
    	    - for changed non-dir REG inode,
    	      data size for its dump is added by bs_blocks * bs_blksize
    	    - for non-changed dir, it sets flag for <pruneneeded>
    	      => we don't want to process this later !
    	  * map_add
    	    - segment = <base, 64-low, 64-mid, 64-high>
    		      = like 64 * 3-bit values (use 0-5)
    		      i.e. for 64 inodes, given start inode number
    		#define MAP_INO_UNUSED  0 /* ino not in use by fs -
                                                 Used for lookup failure */
    		#define MAP_DIR_NOCHNG  1 /* dir, ino in use by fs,
                                                 but not dumped */
    		#define MAP_NDR_NOCHNG  2 /* non-dir, ino in use by fs,
                                                 but not dumped */
    		#define MAP_DIR_CHANGE  3 /* dir, changed since last dump */

    		#define MAP_NDR_CHANGE  4 /* non-dir, changed since last dump */

    		#define MAP_DIR_SUPPRT  5 /* dir, unchanged
                                                 but needed for hierarchy */
    		- hunk = 4 pages worth of segments, max inode#, next ptr in list
    	    - i.e. map = linked list of 4 pages of segments of 64 inode states
        OUTPUT:
    	- inode map = list of all inodes of file system and
    	  for each one there is an associated state variable
    	  describing type of inode and whether it has changed
    	- the inode numbers are stored in chunks of 64
    	  (with only the base inode number explicitly stored)

      - premptchk: progress report, return if got a signal

      - if <pruneneeded> (i.e. non-changed dirs) OR subtrees specified (-s)
        - phase3:  pruning inode map (pruning unneeded subtrees)
    	INPUT:
    	    - subtree list
    	    - inode map
    	FLOW:
    	- bigstat_iter on cb_prune() per inode
    	* cb_prune
    	  - if have subtrees and subtree list contains inode
    	    -> need to traverse every group (inogrp_t) and
                   every page of inode#s
    	    - diriter on cb_count_in_subtreelist
    	      * cb_count_in_subtreelist:
    	      - looks up each inode# (in directory iteration) in subtreelist
    	      - if exists then increment counter
    	    - if at least one inode in list
    	      - diriter on cb_cond_del
    	      * cb_cond_del:
                - TODO

    	OUTPUT:
                - TODO

    - TODO: phase4 and phase5

    - if single-threaded (miniroot or pipeline) then
        * drive_init2
    	- for each drive
    	    * drive_allochdrs
    	    * do_init
        * content_stream_dump
        - return

    - else (multithreaded std. case)
        * drive_init2 (see above)
        * drive_init3
    	- for each drive
    	    * do_sync
        - for each stream create a child manager
    	* cldmgr_create
    	    * childmain
    		* content_stream_dump
    		* do_quit

    - loop waiting for children to die
    * content_complete

##### Dumping to Tape

    * content_stream_dump
      * Media_mfile_begin
        write out global header (includes media header; see below)

      - loop dumping media files
        * inomap_dump()
          - dumps out the linked list of hunks of state maps of inodes

        * dump_dirs()
          - bulkstat through all inodes of file system

          * dump_dir()
            - lookup inode# in inode map
            - if state is UNSUSED or NOCHANGED then skip inode dump
            - jdm_open() = open_by_handle() on directory
            * dump_filehdr()
              - write out 256 padded file header
              - header = <offset, flags, checksum, 128-byte bulk stat structure >
              - bulkstat struct derived from struct xfs_bstat
                - stnd. stat stuff + extent size, #of extents, DMI stuff
              - if HSM context then
                - modify bstat struct to make it offline
            - loops calling getdents()
              - does a bulkstat or bulkstat-single of dir inode
              * dump_dirent()
                - fill in direnthdr_t record
                - <ino, gen & DENTGENMASK, record size,
                      checksum, variable length name (8-char padded)>
                  - gen is from statbuf.bs_gen
                - write out record
            - dump null direnthdr_t record
            - if dumpextattr flag on and it
              has extended attributes (check bs_xflags)
              * dump_extattrs
                * dump_filehdr() with flags of FILEHDR_FLAGS_EXTATTR
                  - for root and non-root attributes
                    - get attribute list (attr_list_by_handle())
                * dump_extattr_list
                  - TODO

        - bigstat iter on dump_file()
          - go thru each inode in file system and apply dump_file
          * dump_file()
    	- if file's inode# is less than the start-point then skip it
    	  -> presume other sproc handling dumping of that inode
    	- if file's inode# is greater than the end-point then stop the loop
    	- look-up inode# in inode map
    	- if not in inode-map OR hasn't changed then skip it
    	- elsif stat is NOT a non-dir then we have an error
    	- if have an hsm context then initialize context
    	- call dump function depending on file type (S_IFREG, S_IFCHR, etc.)

    	  * dump_file_reg (for S_IFREG):
    	    -> see below

    	  * dump_file_spec (for S_IFCHAR|BLK|FIFO|NAM|LNK|SOCK):
    	    - dump file header
    	    - if file is S_IFLNK (symlink) then
    	      - read link by handle into buffer
    	      - dump extent header of type, EXTENTHDR_TYPE_DATA
    	      - write out link buffer (i.e. symlink string)

    	  - if dumpextattr flag on and it
    	    has extended attributes (check bs_xflags)
    	    * dump_extattrs (see the same call in the dir case above)

        - set mark

        - if haven't hit EOM (end of media) then
          - write out null file header
          - set mark

        - end media file by do_end_write()

        - if got an inventory stream then
          * inv_put_mediafile
    	- create an inventory-media-file struct (invt_mediafile_t)
    	  - < media-obj-id, label, index, start-ino#, start-offset,
    		 end-ino#, end-offset, size = #recs in media file, flag >
    	* stobj_put_mediafile

      - end of loop of media file dumping
      - lock and increment the thread done count

      - if dump supports multiple media files (tapes do but dump-files don't) then
        - if multi-threaded then
          - wait for all threads to have finished dumping
            (loops sleeping for 1 second each iter)
        * dump_session_inv
          * inv_get_sessioninfo
            (get inventory session data buffer)
            * stobj_get_sessinfo
            * stobj_pack_sessinfo
          * Media_mfile_begin
          - write out inventory buffer
          * Media_mfile_end
          * inv_put_mediafile (as described above)
        * dump_terminator
          * Media_mfile_begin
          * Media_mfile_end

    * dump_file_reg (for S_IFREG):
      - if this is the start inode, then set the start offset
      - fixup offset for resumed dump
      * init_extent_group_context
        - init context - reset getbmapx struct fields with offset=0, len=-1
        - open file by handle
        - ensure Mandatory lock not set
      - loop dumping extent group
        - dump file header
        * dump_extent_group() [content.c]
          - set up realtime I/O size
          - loop over all extents
    	- dump extent
    	  - stop if we reach stop-offset
    	  - stop if offset is past file size i.e. reached end
    	  - stop if exceeded per-extent size

    	  - if next-bmap is at or past end-bmap then get a bmap
    	    - fcntl( gcp->eg_fd, F_GETBMAPX, gcp->eg_bmap[] )
    	    - if have an hsm context then
    	      - call HsmModifyExtentMap()
    	    - next-bmap = eg_bmap[1]
    	    - end-bmap = eg_bmap[eg_bmap[0].bmv_entries+1]

    	  - if bmap entry is a hole (bmv_block == -1) then
    	    - if dumping ext.attributes then
    	      - dump extent header with bmap's offset,
    		extent-size and type EXTENTHDR_TYPE_HOLE

    	    - move onto next bmap
    	      - if bmap's (offset + len)*512 > next-offset then
    		update next-offset to this
    	      - inc ptr

    	  - if bmap entry has zero length then
    	    - move onto next bmap

    	  - get extsz and offset from bmap's bmv_offset*512 and bmv_length*512

    	  - about 8 different conditions to test for
    	    - cause function to return OR
    	    - cause extent size to change OR...

    	  - if realtime or extent at least a PAGE worth then
    	    - align write buffer to a page boundary
    	    - dump extent header of type, EXTENTHDR_TYPE_ALIGN

    	  - dump extent header of type, EXTENTHDR_TYPE_DATA
    	  - loop thru extent data to write extsz worth of bytes
    	    - ask for a write buffer of extsz but get back actualsz
    	    - lseek to offset
    	    - read data of actualsz from file into buffer
    	    - write out buffer
    	    - if at end of file and have left over space in the extent then
    	      - pad out the rest of the extent
    	    - if next offset is at or past next-bmap's offset+len then
    	      - move onto next bmap
        - dump null extent header of type, EXTENTHDR_TYPE_LAST
        - update bytecount and media file size
      - close the file

#### Splitting a Regular File

If a regular file is greater than 16Mb (maxextentcnt = drivep->d_recmarksep = recommended max. separation between marks), then it is broken up into multiple extent groups each with their own filehdr_t's. A regular file can also be split, if we are dumping to multiple streams and the file would span the stream boundary.

#### Splitting a dump over multiple streams (Phase 5)

If one is dumping to multiple streams, then xfsdump calculates an estimate of the dump size and divides by the number of streams to determine how much data we should allocate for a stream. The inodes are processed in order from bulkstat in the function cb_startpt. Thus we start allocating inodes to the first stream until we reach the allocated amount and then need to decide how to proceed on to the next stream. At this point we have 3 actions:

Hold
  Include this file in the current stream.
Bump
  Start a new stream beginning with this file.
Split
  Split this file across 2 streams in different extent groups.

### xfsrestore

#### Control Flow of xfsrestore

content_init (xfsrestore version)

Initialize the mmap files of:

- "$dstdir/xfsrestorehousekeepingdir/state"
- "$dstdir/xfsrestorehousekeepingdir/dirattr"
- "$dstdir/xfsrestorehousekeepingdir/dirextattr"
- "$dstdir/xfsrestorehousekeepingdir/namreg"
- "$dstdir/xfsrestorehousekeepingdir/inomap"
- "$dstdir/xfsrestorehousekeepingdir/tree"

content_stream_restore

- one stream does while others wait:
  - validates command line dump spec against the online inventory
  - incorporates the online inventory into the persistent inventory
- one stream does while others wait:
  - if which session to restore is still unknown then
    - search media files of dump to match command args or ask the user to select the media file
    - add found media file to persistent inventory
- one stream does while others wait:
  - search for directory dump
  - calls dirattr_init if necessary
  - calls namreg_init if necessary
  - initialize the directory tree (tree_init)
  - read the dirents into the tree (applydirdump)
- one stream does while others wait:
  - do tree post processing (treepost)
    - create the directories (mkdir)
    - cumulative restore file system fixups
- all threads can process each media file of their dumps for restoring the non-directory files
  - loop over each media file
    - read in file header
    - call applynondirdump for file hdr
      - restore extended attributes for file (if it is last extent group of file)
      - restore file
        - loop thru all hardlink paths from tree for inode (tree_cb_links) and call restore_file_cb
          - if a hard link then link(path1, path2)
          - else restore the non-dir object:
            - S_IFREG -> restore_reg - restore regular file
              - truncate file to bs_size
              - set the bs_xflags for extended attributes
              - set DMAPI fields if necessary
              - loop processing the extent headers
                - if type LAST then exit loop
                - if type ALIGN then eat up the padding
                - if type HOLE then ignore
                - if type DATA then copy the data into the file for the extent; seeking to extent start if necessary
              - register the extent group in the partial registry
              - set timestamps using utime(2)
              - set permissions using fchmod(2)
              - set owner/group using fchown(2)
            - S_IFLNK -> restore_symlink
            - else -> restore_spec
        - if no hardlinks references for inode in tree then restore file into orphanage directory
      - update stats
      - loop
        - get mark
        - read file header
        - if corrupt then go to next mark
        - else exit loop
- one stream does while others wait:
  - finalize
    - restore directory attributes
    - remove orphanage directory
    - remove persistent inode map

content_init in a bit more detail(xfsrestore version)

- create house-keeping-directory for persistent mmap file data structures. For cumulative and interrupted restores, we need to keep restore session data between invocations of xfsrestore.
- mmap the "state" file and create if not already existing. Initially just mmap the header. (More details below)
- if continuing interrupted session then
  - initialize and mmap the directory attribute data and dirextattr file (dirattr_init)
  - initialize name registry data (namreg_init)
  - initialize and mmap the inode map (inomap_sync_pers)
  - initialize and mmap the dirent tree (tree_sync)

  - finalize -> restore directory attributes, delete inode map
- mmap the state file for the header and the subtree selections
- update the state header with the command line predicates
- update the subtree selections via the -s option
- create extended attribute buffers for each stream
- mmap the state file for the persistent inventory descriptors

- initialize and mmap the directory attribute data and dirextattr file (dirattr_init)
- initialize name registry data (namreg_init)
- initialize and mmap the inode map (inomap_sync_pers)
- initialize and mmap the dirent tree (tree_sync)

#### Persistent Inventory and State File

The persistent inventory is found inside the "state" file. The state file is an mmap'ed file called $dstdir/xfsrestorehousekeepingdir/state. The state file (struct pers from content.c) contains a header of:

- command line arguments from 1st session,
- partial registry data structure for use with multiple streams and extended attributes,
- various session state such as dumpid, dump label, number of inodes restored so far, etc.

Followed by pages for the subtree selections and then the persistent inventory.
So the 3 main sections look like:

    "state" mmap file
    ---------------------
    | State Header      |
    | (number of pages  |
    |  to hold pers_t)  |
    | pers_t:           |
    | accum. state      |
    |   - cmd opts      |
    |   - etc...        |
    | session state     |
    |   - dumpid        |
    |   - accum.time    |
    |   - ino count     |
    |   - etc...        |
    |   - stream head   |
    ---------------------
    | Subtree           |
    | Selections        |
    | (stpgcnt * pgsz)  |
    ---------------------
    | Persistent        |
    | Inventory         |
    | Descriptors       |
    | (descpgcnt * pgsz)|
    |                   |
    ---------------------

Persistent Inventory Tree

    e.g. drive1         drive2        drive3
    |-------------|  |---------|   |---------|
    | stream1     |->| stream2 |-->| stream3 |
    |(pers_strm_t)|  |         |   |         |
    |-------------|  |---------|   |---------|
    		    ||
    		    \/
                     e.g. tape21        tape22         tape23
    		 |------------|   |---------|   |---------|
    		 |  obj1      |-->|  obj2   |-->|  obj3   |
    		 |(pers_obj_t)|   |         |   |         |
    		 |------------|   |---------|   |---------|
    				    ||
    				    \/
    				 |-------------|   |---------|   |---------|
    				 | file1       |-->|  file2  |-->|  file3  |
    				 |(pers_file_t)|   |         |   |         |
    				 |-------------|   |---------|   |---------|

[TODO: persistent inventory needs investigation]

#### Restore's directory entry tree

As can be seen in the directory dump format above, part of the dump consists of directories and their associated directory entries. The other part consists of the files which are just identified by their inode# which is sourced from bulkstat during the dump. When restoring a dump, the first step is reconstructing the tree of directory nodes. This tree can then be used to associate the file with it's directory and so restored to the correct location in the directory structure.

The tree is an mmap'ed file called $dstdir/xfsrestorehousekeepingdir/tree. Different sections of it will be mmap'ed separately. It is of the following format:

    --------------------
    |  Tree Header     | <--- ptr to root of tree, hash size,...
    |  (pgsz = 16K)    |
    --------------------
    |  Hash Table      | <--- inode# ==map==> tree node
    --------------------
    |  Node Header     | <--- describes allocation of nodes
    |  (pgsz = 16K)    |
    --------------------
    |  Node Segment#1  | <--- typically 1 million tree nodes
    --------------------
    |  ...             |
    |                  |
    --------------------
    |  Node Segment#N  |
    --------------------

The tree header is described by restore/tree.c/treePersStorage, and it has such things as pointers to the root of the tree and the size of the hash table.

            ino64_t p_rootino - ino of root
            nh_t p_rooth - handle of root node
            nh_t p_orphh - handle to orphanage node
            size64_t p_hashsz - size of hash array
            size_t p_hashmask - hash mask (private to hash abstraction)
            bool_t p_ownerpr - whether to restore directory owner/group attributes
            bool_t p_fullpr - whether restoring a full level 0 non-resumed dump
            bool_t p_ignoreorphpr - set if positive subtree or interactive

The hash table maps the inode number to the tree node. It is a chained hash table with the "next" link stored in the tree node in the n_hashh field of struct node in restore/tree.c. The size of the hash table is based on the number of directories and non-directories (which will approximate the number of directory entries - won't include extra hard links). The size of the table is capped below at 1 page and capped above at virtual-memory-limit/4/8 (i.e. vmsz/32) or the range of 2^32 whichever is the smaller.

The node header is described by restore/node.c/node_hdr_t and it contains fields to help in the allocation of nodes.

            size_t nh_nodesz -  internal node size
            ix_t nh_nodehkix -
            size_t nh_nodesperseg - num nodes per segment
            size_t nh_segsz - size in bytes of segment
            size_t nh_winmapmax - maximum number of windows
                                  based on using up to vmsz/4
            size_t nh_nodealignsz - node alignment
            nix_t nh_freenix - pointer to singly linked freelist
            off64_t nh_firstsegoff - offset to 1st segment
            off64_t nh_virgsegreloff - (see diagram)
                     offset (relative to beginning of first segment) into
                     backing store of segment containing one or
                     more virgin nodes. relative to beginning of segmented
                     portion of backing store. bumped only when all of the
                     nodes in the segment have been placed on the free list.
                     when bumped, nh_virginrelnix is simultaneously set back
                     to zero.
            nix_t nh_virgrelnix - (see diagram)
                     relative node index within the segment identified by
                     nh_virgsegreloff of the next node not yet placed on the
                     free list. never reaches nh_nodesperseg: instead set
                     to zero and bump nh_virgsegreloff by one segment.

All the directory entries are stored in a node segment. Each segment holds around 1 million nodes (NODESPERSEGMIN). The value is greater because the size in bytes must be a multiple of the node size and the page size. However, the code handling the number of nodes was changed recently due to problems at a site. The number of nodes is now based on the value of dircnt+nondircnt in an attempt to fit most of the entries into 1 segment. As the value of dircnt+nondircnt is an approximation to the number of directory entries, we cap below at 1 million entries as was done previously.

Each segment is mmap'ed separately. In fact, the actual allocation of nodes is handled by a few abstractions. There is a node abstraction and a window abstraction. At the node abstraction when one wants to allocate a node using node_alloc(), one first checks the free-list of nodes. If the free list is empty then a new window is mapped and a chunk of 8192 nodes are put on the free list by linking each node using the first 8 bytes (ignoring node fields).

      SEGMENT (default was about 1 million nodes)
    |----------|
    | |------| |
    | |      | |
    | | 8192 | |
    | | nodes| |   nodes already used in tree
    | | used | |
    | |      | |
    | |------| |
    |          |
    | |------| |
    | |   --------| <-----nh_freenix (ptr to node-freelist)
    | |node1 | |  |
    | |------| |  | node-freelist (linked list of free nodes)
    | |   ----<---|
    | |node2 | |
    | |------| |
    ............
    |----------|

##### Window Abstraction

The window abstraction manages the mapping and unmapping of the segments (of nodes) of the dirent tree. In the node allocation, mentioned above, if our node-freelist is empty we call win_map() to map in a chunk of 8192 nodes for the node-freelist.

Consider the win_map(offset, return_memptr) function:

    One is asking for an offset within a segment.
    It looks up its bag for the segment (given the offset), and
    if it's already mapped then
        if the window has a refcnt of zero, then remove it from the win-freelist
        it uses that address within the mmap region and
        increments refcnt.
    else if it's not in the bag then
        if win-freelist is not empty then
            munmap the oldest mapped segment
    	remove head of win-freelist
            remove the old window from the bag
        else /* empty free-list */
            allocate a new window
        endif
        mmap the segment
        increment refcnt
        insert window into bag of mapped segments
    endif

The window abstraction maintains an LRU win-freelist not to be confused with the node-freelist. The win-freelist consists of windows (stored in a bag) which are doubly linked ordered by the time they were used. Whereas the node-freelist, is used to get a new node in the node allocation.

Note that the windows are stored in 2 lists. They are doubly linked in the LRU win-freelist and are also stored in a bag. A bag is just a doubly linked searchable list where the elements are allocated using calloc(). It uses the bag as a container of mmaped windows which can be searched using the bag key of window-offset.

    BAG:  |--------|     |--------|     |--------|     |--------|     |-------|
          | win A  |<--->| win B  |<--->| win C  |<--->| win D  |<--->| win E |
          | ref=2  |     | ref=1  |     | ref=0  |     | ref=0  |     | ref=0 |
          | offset |     | offset |     | offset |     | offset |     | offset|
          |--------|     |--------|     |--------|     |--------|     |-------|
                                          ^                               ^
                                          |                               |
                                          |                               |
                         |----------------|       |-----------------------|
    LRU             |----|---|               |----|---|
    win-freelist:   | oldest |               | 2nd    |
                    | winptr |<------------->| oldest |<----....
                    |        |               | winptr |
                    |--------|               |--------|

Call Chain
Below are some call chain scenarios of how the allocation of dirent tree nodes are done at different stages.

    1st time we allocate a dirent node:

    applydirdump()
      Go thru each directory entry (dirent)
        tree_addent()
          if new entry then
             Node_alloc()
               node_alloc()
                 win_map()
                   mmap 1st segment/window
                   insert win into bag
    	       refcnt++
                 make node-freelist of 8192 nodes (linked list)
                 remove list node from freelist
                 win_unmap()
                   refcnt--
                   put win on win-freelist (as refcnt==0)
                 return node

    2nd time we call tree_addent():

          if new entry then
             Node_alloc()
               node_alloc()
                 get node off node-freelist (8190 nodes left now)
                 return node

    8193th time when we have used up 8192 nodes and node-freelist is emtpy:

          if new entry then
             Node_alloc()
               node_alloc()
                 there is no node left on node-freelist
                 win_map at the address after the old node-freelist
                   find this segment in bag
                     refcnt==0, so remove from LRU win-freelist
                     refcnt++
                     return addr
                 make a node-freelist of 8192 nodes from where left off last time
                 win_unmap
                   refcnt--
                   put on LRU win-freelist as refcnt==0
                 get node off node-freelist (8191 nodes left now)
                 return node

    When whole segment used up and thus all remaining node-freelist
    nodes are gone then
    (i.e. in old scheme would have used up all 1 million nodes
     from first segment):

          if new entry then
             Node_alloc()
               node_alloc()
                 if no node-freelist then
                   win_map()
                     new segment not already mapped
                     LRU win-freelist is not empty (we have 1st segment)
                     remove head from LRU win-freelist
                     remove win from bag
                     munmap its segment
                     mmap the new segment
                     add to bag
                     refcnt++
                   make a new node-freelist of 8192 nodes
                   win_unmap()
                     refcnt--
                     put on LRU win-freelist as refcnt==0
                   get node off node-freelist (8191 nodes left now)
                   return node

Pseudo-code of snippets of directory tree creation functions (from notes) gives one an idea of the flow of control for processing dirents and adding to the tree and other auxiliary structures:

    content_stream_restore()
      ...
      Get next media file
      dirattr_init() - initialize directory attribute structure
      namereg_init() - initialize name registry structure
      tree_init() - initialize dirent tree
      applydirdump() - process the directory dump and create tree - see below
      treepost() - tree post processing where mkdirs happen
      ...

    applydirdump()
      ...
      inomap_restore_pers() - read ino map
      read directories and their entries
        loop 'til null hdr
           dirh = tree_begindir(fhdr, dah) - process dir filehdr
           loop 'til null entry
             rv = read_dirent()
             tree_addent(dirh, dhdrp->dh_ino, dh_gen, dh_name, namelen)
           endloop
           tree_enddir(dirh)
        endloop
      ...

    tree_beginddir(fhdrp - fileheader, dahp - dirattrhandle)
      ...
      ino = fhdrp->fh_stat.bs_ino
      hardh = link_hardh(ino, gen) - lookup inode in tree
      if (hardh == NH_NULL) then
        new directory - 1st time seen
        dah = dirattr_add(fhdrp) - add dir header to dirattr structure
        hardh = Node_alloc(ino, gen,....,NF_ISDIR|NF_NEWORPH)
        link_in(hardh) - link into tree
        adopt(p_orphh, hardh, NRH_NULL) - put dir in orphanage directory
      else
        ...
      endif

    tree_addent(parent, inode, size, name, namelen)
      hardh = link_hardh(ino, gen)
      if (hardh == NH_NULL) then
        new entry - 1st time seen
        nrh = namreg_add(name, namelen)
        hardh = Node_alloc(ino, gen, NRH_NULL, DAH_NULL, NF_REFED)
        link_in(hardh)
        adopt(parent, hardh, nrh)
      else
        ...
      endif

#### Cumulative Restore

A cumulative restore seems a bit different than one might expect. It tries to restore the state of the filesystem at the time of the incremental dump. As the man page states: "This can involve adding, deleting, renaming, linking, and unlinking files and directories." From a coding point of view, this means we need to know what the dirent tree was like previously compared with what the dirent tree is like now. We need this so we can see what was added and deleted. So this means that the dirent tree, which is stored as an mmap'ed file in restoredir/xfsrestorehousekeepingdir/tree should not be deleted between cumulative restores (as we need to keep using it).

So on the first level 0 restore, the dirent tree is created. When the directories are restored and the files are restored, the corresponding tree nodes are marked as NF_REAL. On the next level cumulative restore, when it is processing the dirents, it looks them up in the tree (created on previous restore). If the entry alreadys exists then it marks it as NF_REFED.

In case a dirent has gone away between times of incremental dumps, xfsrestore does an extra pass in the tree preprocessing which traverses the tree looking for non-referenced (not NF_REFED) nodes so that if they exist in the FS (i.e. are NF_REAL) then they can be deleted (so that the FS resembles what it was at the time of the incremental dump). Note there are more conditionals to the code than just that - but that is the basic plan. It is elaborated further below.

#### Cumulative Restore Tree Postprocessing

After the dirent tree is created or updated from the directory dump cumulative restoral, it does a 4 step postprocessing (treepost):

Steps of Tree Postprocessing
Function | What it does
--- | ---
1. noref_elim_recurse | remove deleted dirs rename moved dirs to orphanage remove extra deleted hard links rename moved non-dirs to orphanage
2. mkdirs_recurse | mkdirs on (dir & !real & ref & sel)
3. rename_dirs | rename moved dirs from orphanage to destination
4. proc_hardlinks | rename moved non-dirs from orphanage to destination remove deleted non-dirs (real & !ref & sel) create a link on rename error (don't understand this one)

Step 1 was changed so that files which are deleted and not moved are deleted early on, otherwise, it can stop a parent directory from being deleted. The new step is:

Function | What it does
--- | ---
1. noref_elim_recurse | remove deleted dirs rename moved dirs to orphanage remove extra deleted hard links rename moved non-dirs to orphanage remove deleted non-dirs which aren't part of a rename

One will notice that renames are not performed directly. Instead entries are renamed to the orphanage, directories are created, then entries are moved from the orphanage to the intended destination. This would be done as renames may not succeed until directories are created. And the directories are not created first as we may be able to create the entry by just moving an existing one. The step of "removing deleted non-dirs" in proc_hardlinks should not happen now since it is done earlier.

#### Partial Registry

The partial registry is a data structure used in xfsrestore for ensuring that files which have been split into multiple extent groups, do not restore the extended attributes until the entire file has been restored. The reason for this is apparently so that DMAPI attributes aren't restored until we have the complete file. Each extent group dumped has the identical copy of the extended attributes (EAs) for that file, thus without this data-structure we could apply the first EAs we come across.

The data structure is of the form:

    Array of M entries:
    -------------------
    0: inode#
       Array for each drive
         drive1:
         ...
         driveN:
    -------------------
    1: inode#
       Array for each drive
    -------------------
    2: inode#
       Array for each drive
    -------------------
    ...
    -------------------
    M-1: inode#
         Array for each drive
    -------------------

    Where N = number of drives (streams); M = 2 * N - 1

There can only be 2*N-1 entries for the partial registry because each stream can contribute an entry for its current inode and one for a previous inode which is split - except for the 1st inode which cannot have a previous split.

          stream 1        stream 2         stream 3      ...  stream N
      |---------------|----------------|-------------------|------------|
      |            ------   -----   ------   -----      -------  -----  |
      |            C  | P     C        | P     C           |  P    C    |
      |---------------|----------------|-------------------|------------|

           current       prev.+curr.        prev.+curr.      prev.+curr.

    Where C = current; P = previous

So if an extent group is processed which doesn't cover the whole file, then the extent range for this file is updated with the partial registry. If the file doesn't exist in the array then a new entry is added. If the file does exist in the array then the extent group for the given drive is updated. It is worth remembering that one drive (stream) can have multiple extent groups (if it is >16Mb) in which case the extent group is just extended (they are split up in order).

A bug was discovered in this area of code, for DMF offline files which have an associated file size but no data blocks allocated and thus no extents. The Offline files were wrongly added to the partial registry because on restore they did not complete the size of the file (because they are offline!). These types of files which do not restore data are now special cased.

### Drive Strategies

The I/O which happens when reading and writing the dump can be to a tape, file, stdout or to a tape remotely via rsh(1) (or $RSH) and rmt(1) (or $RMT). There are 3 pieces of code called strategies which handle the dump I/O:

- drive_scsitape
- drive_minrmt
- drive_simple

There is an associated data structure - below is one for drive_scsitape:

        drive_strategy_t drive_strategy_scsitape = {
    	    DRIVE_STRATEGY_SCSITAPE,        /* ds_id */
    	    "scsi tape (drive_scsitape)",   /* ds_description */
    	    ds_match,                       /* ds_match */
    	    ds_instantiate,                 /* ds_instantiate */
    	    0x1000000ll,                    /* ds_recmarksep  16 MB */
    	    0x10000000ll,                   /* ds_recmfilesz 256 MB */
        };

The choice of the strategy to use is done by a scoring scheme which is probably not warranted IMHO. (A direct cmd option would be simpler and less confusing.) The scoring function is called ds_match.

strategy | IRIX scoring | Linux scoring
--- | --- | ---
drive_scsitape | score badly with -10 if: stdio pathname if colon (':') in pathname (assumes remote) and open on pathname fails MTIOCGET ioctl fails or not colon and drivername is not "tpsc" or "ts_" else if syscalls complete ok then we score 10. | score like IRIX but instead of checking drivername associated with path (not available on Linux), score -10 if the following: stat fails it is not a character device its real path does not contain "/nst", "/st" nor "/mt".
drive_minrmt | score badly with -10 if stdio pathname score 10 if have all of the following: colon is in the pathname (assumes remote from this) blocksize set with -b option minrmt chosen with -m option otherwise score badly with -10 | score like IRIX but do not require a colon in the pathname; i.e. one can use this strategy on Linux without requiring a remote pathname
drive_simple | score badly with -1 if stat fails on local pathname pathname is a local directory otherwise score with 1 | identical to IRIX

Each strategy is organised like a "class" with functions/methods in the data structure:

            do_init,
            do_sync,
            do_begin_read,
            do_read,
            do_return_read_buf,
            do_get_mark,
            do_seek_mark,
            do_next_mark,
            do_end_read,
            do_begin_write,
            do_set_mark,
            do_get_write_buf,
            do_write,
            do_get_align_cnt,
            do_end_write,
            do_fsf,
            do_bsf,
            do_rewind,
            do_erase,
            do_eject_media,
            do_get_device_class,
            do_display_metrics,
            do_quit,

#### Drive Scsitape

This strategy is the main one used for dumps to tape and dumps to a remote tape. This strategy on IRIX can be used for remote dumps to another IRIX machine. On Linux, this strategy is used for remote dumps to Linux or IRIX machines. Remote dumping uses the librmt library, see below.

If xfsdump/xfsrestore is running single-threaded (-Z option) or is running on Linux (which is not multi-threaded) then records are read/written straight to the tape. If it is running multi-threaded then a circular buffer is used as an intermediary between the client and worker threads.

Initially drive_init1() calls ds_instantiate() which if dump/restore is running multi-threaded, creates the ring buffer with ring_create which initialises the state to RING_STAT_INIT and sets up the worker thread with ring_worker_entry.

    ds_instantiate()
      ring_create(...,ring_read, ring_write,...)
        - allocate and init buffers
        - set rm_stat = RING_STAT_INIT
        start up worker thread with ring_worker_entry

The worker spends its time in a loop getting items from the active queue, doing the read or write operation and placing the result back on the ready queue.

    worker
    ======
    ring_worker_entry()
      loop
        ring_worker_get() - get from active queue
        case rm_op
          RING_OP_READ -> ringp->r_readfunc
          RING_OP_WRITE -> ringp->r_writefunc
          ..
        endcase
        ring_worker_put() - puts on ready queue
      endloop

##### Reading

Prior to reading, one needs to call do_begin_read(), which calls prepare_drive(). prepare_drive() opens the tape drive if necessary and gets its status. It then works out the tape record size to use (set_best_blk_and_rec_sz) using current max blksize (mtinfo.maxblksz from ioctl(fd,MTIOCGETBLKINFO,minfo)) on the scsi tape device in IRIX.

On IRIX (from set_best_blk_and_rec_sz):

- local tape -> tape_recsz = min(STAPE_MAX_RECSZ = 2 Mb, mtinfo.maxblksz)
  which typically would mean 2 Mb.
- remote tape -> tape_recsz = STAPE_MIN_MAX_BLKSZ = 240 Kb

On Linux:

- local tape ->
  - tape_recsz = STAPE_MAX_LINUX_RECSZ = 1 Mb
  - or if -b cmdlineblksize specified then
    tape_recsz = min(STAPE_MAX_RECSZ = 2 Mb, cmdlineblksize)
    which typically would mean cmdlineblksize.
- remote tape -> tape_recsz = STAPE_MIN_MAX_BLKSZ = 240 Kb

If we have a fixed size device, then it tries to read initially at minimum(2Mb, current max blksize) but if it reads in a smaller number of bytes than this, then it will try again for STAPE_MIN_MAX_BLKSZ = 240 Kb data.

    prepare_drive()
      open drive (repeat & timeout if EBUSY)
      get tape status (repeat 'til timeout or online)
      set up tape rec size to try
      loop trying to read a record using straight Read()
          if variable blksize then
    	 ok = nread>0 & !EOD & !EOT & !FileMark
          else fixed blksize then
    	 ok = nread==tape_recsz & !EOD & !EOT & !FileMark
          endif
          if ok then
    	validate_media_file_hdr()
          else
            could be an error or try again with newsize
            (complicated logic in this code!)
          endif
      endloop

For each do_read call in the multi-threaded case, we have two sides to the story: the client which is coming from code in content.c and the worker which is a simple thread just satisfying I/O requests. From the point of view of the ring buffer, these are the steps which happen for reading:

1. client removes msg from ready queue
2. client wants to read, so sets op field to READ (RING_OP_READ) and puts on active queue
3. worker removes msg from active queue, invokes client read function, sets status field: OK/ERROR, puts msg on ready queue
4. client removes this msg from ready queue

The client read code looks like the following:

    client
    ======
    do_read()
      getrec()
        singlethreaded -> read_record() -> Read()
        else ->
          loop 'til contextp->dc_recp is set to a buffer
    	Ring_get() -> ring.c/ring_get()
    	  remove msg from ready queue
    	      block on ready queue - qsemP( ringp->r_ready_qsemh )
    	      msgp = &ringp->r_msgp[ ringp->r_ready_out_ix ];
    	      cyclic_inc(ringp->r_ready_out_ix)
            case rm_stat:
    	  RING_STAT_INIT, RING_STAT_NOPACK, RING_STAT_IGNORE
                put read msg on active queue
    		contextp->dc_msgp->rm_op = RING_OP_READ
    		Ring_put(contextp->dc_ringp,contextp->dc_msgp);
              RING_STAT_OK
                contextp->dc_recp = contextp->dc_msgp->rm_bufp
              ...
            endcase
          endloop

#### Librmt

Librmt is a standard library on IRIX which provides a set of remote I/O functions:

- rmtopen
- rmtclose
- rmtioctl
- rmtread
- rmtwrite

On linux, a librmt library is provided as part of the xfsdump distribution. The remote functions are used to dump/restore to remote tape drives on remote machines. It does this by using rsh or ssh to run rmt(1) on the remote machine. The main caveat, however, comes into play for the rmtioctl function. Unfortunately, the values for mt operations and status codes are different on different machines. For example, the offline command op on IRIX is 6 and on Linux it is 7. On Linux, 6 is rewind and on IRIX 7 is a no-op. So for the Linux xfsdump, the rmtiocl function has been rewritten to check what the remote OS is (e.g. rsh host uname) and do appropriate mappings of codes. As well as the different mt op codes, the mtget structures differ for IRIX and Linux and for Linux 32 bit and Linux 64 bit. The size of the mtget structure is used to determine which structure it is and the value of mt_type is used to determine if endian conversion needs to be done.

#### Drive Minrmt

The minrmt strategy was written based (copied) on the scsitape strategy. It has been simplified so that the state of the tape driver is not needed (i.e. status of EOT, BOT, EOD, FMK,... are not used) and the current blk size of the tape driver is not used. Instead error handling is based on the return codes from reading and writing and the blksize must be give as a parameter. It was designed for talking to remote NON-IRIX hosts where the status codes can vary. However, as was mentioned in the discussion of librmt on Linux, the mt operations vary on foreign hosts as well as the status codes. So this is only a limited solution.

#### Drive Simple

The simple strategy was designed for dumping to files or stdout. It is simpler in that it does NOT have to worry about:

- the ring buffer
- talking to the scsitape driver with various operations and status
- multiple media files

### Online Inventory

xfsdump keeps a record of previous xfsdump executions in the online inventory stored in /var/xfsdump/inventory or for Linux, /var/lib/xfsdump/inventory. This inventory is used to determine which previous dump a incremental dump should be based on. That is, when doing a level > 0 dump for a filesystem, xfsdump will refer to the online inventory to work out when the last dump for that filesystem was performed in order to work out which files will be included in the current dump. I believe the online inventory is also used by xfsrestore in order to determine which tapes will be needed to completely restore a dump.

xfsinvutil is a utility originally designed to remove unwanted information from the online inventory. Recently it has been beefed up to allow interactive browsing of the inventory and the ability to merge/import one inventory into another. (See Bug 818332.)

The inventory consists of three types of files:

Inventory files
Filename | Description
--- | ---
fstab | There is one fstab file which contains the list of filesystems that are referenced in the inventory.
*.InvIndex | There is one InvIndex file per filesystem which contain pointers to the StObj files sorted temporaly.
*.StObj | There may be many StObj files per filesystem. Each file contains information about, up to five, individual xfsdump executions. The information relates to what tapes were used, which inodes are stored in which media files, etc.

The files are constructed like so:

#### fstab

fstab structure
Quantity | Data structure
--- | ---
1 |
    typedef struct invt_counter {
        INVT_COUNTER_FIELDS
            uint32_t      ic_vernum;/* on disk version number for posterity */\
            u_int         ic_curnum;/* number of sessions/invindices recorded \
                                       so far */                              \
            u_int         ic_maxnum;/* maximum number of sessions/inv_indices \
                                       that we can record on this stobj */

        char              ic_padding[0x20 - INVT_COUNTER_FIELDS_SIZE];
    } invt_counter_t;
1 per filesystem |
    typedef struct invt_fstab {
        uuid_t  ft_uuid;
        char    ft_mountpt[INV_STRLEN];
        char    ft_devpath[INV_STRLEN];
        char    ft_padding[16];
    } invt_fstab_t;

#### InvIndex

InvIndex structure
Quantity | Data structure
--- | ---
1 |
    typedef struct invt_counter {
        INVT_COUNTER_FIELDS
            uint32_t      ic_vernum;/* on disk version number for posterity */\
            u_int         ic_curnum;/* number of sessions/invindices recorded \
                                       so far */                              \
            u_int         ic_maxnum;/* maximum number of sessions/inv_indices \
                                       that we can record on this stobj */
        char              ic_padding[0x20 - INVT_COUNTER_FIELDS_SIZE];
    } invt_counter_t;
1 per StObj file |
    typedef struct invt_entry {
        invt_timeperiod_t ie_timeperiod;
        char              ie_filename[INV_STRLEN];
        char              ie_padding[16];
    } invt_entry_t;

#### StObj

StObj structure
Quantity | Data structure
--- | ---
1 |
    typedef struct invt_sescounter {
        INVT_COUNTER_FIELDS
            uint32_t      ic_vernum;/* on disk version number for posterity */\
            u_int         ic_curnum;/* number of sessions/invindices recorded \
                                       so far */                              \
            u_int         ic_maxnum;/* maximum number of sessions/inv_indices \
                                       that we can record on this stobj */
        off64_t  ic_eof;   /* current end of the file, where the next
                              media file or stream will be written to */
        char     ic_padding[0x20 - ( INVT_COUNTER_FIELDS_SIZE + sizeof( off64_t) )];
    } invt_sescounter_t;
fixed space for INVT_STOBJ_MAXSESSIONS (ie. 5) |
    typedef struct invt_seshdr {
        off64_t    sh_sess_off;    /* offset to rest of the sessioninfo */
        off64_t    sh_streams_off; /* offset to start of the set of
                                      stream hdrs */
        time_t     sh_time;        /* time of the dump */
        uint32_t   sh_flag;        /* for misc flags */
        u_char     sh_level;       /* dump level */
        u_char     sh_pruned;      /* pruned by invutil flag */
        char       sh_padding[22];
    } invt_seshdr_t;
fixed space for INVT_STOBJ_MAXSESSIONS (ie. 5) |
    typedef struct invt_session {
        uuid_t   s_sesid;	/* this session's id: 16 bytes*/
        uuid_t   s_fsid;	/* file system id */
        char     s_label[INV_STRLEN];  /* session label */
        char     s_mountpt[INV_STRLEN];/* path to the mount point */
        char     s_devpath[INV_STRLEN];/* path to the device */
        u_int    s_cur_nstreams;/* number of streams created under
                                   this session so far */
        u_int    s_max_nstreams;/* number of media streams in
                                   the session */
        char     s_padding[16];
    } invt_session_t;
any number |
    typedef struct invt_stream {
        /* duplicate info from mediafiles for speed */
        invt_breakpt_t  st_startino;   /* the starting pt */
        invt_breakpt_t  st_endino;     /* where we actually ended up. this
                                          means we've written upto but not
                                          including this breakpoint. */
        off64_t         st_firstmfile;  /*offsets to the start and end of*/
        off64_t         st_lastmfile;	  /* .. linked list of mediafiles */
        char            st_cmdarg[INV_STRLEN]; /* drive path */
        u_int           st_nmediafiles; /* number of mediafiles */
        bool_t          st_interrupted;	/* was this stream interrupted ? */
        char            st_padding[16];
    } invt_stream_t;
    typedef struct invt_mediafile {
        uuid_t           mf_moid;	    /* media object id */
        char             mf_label[INV_STRLEN];	/* media file label */
        invt_breakpt_t   mf_startino; /* file that we started out with */
        invt_breakpt_t   mf_endino;	  /* the dump file we ended this
                                         media file with */
        off64_t          mf_nextmf;   /* links to other mfiles */
        off64_t          mf_prevmf;
        u_int            mf_mfileidx; /* index within the media object */
        u_char           mf_flag;     /* Currently MFILE_GOOD, INVDUMP */
        off64_t          mf_size;     /* size of the media file */
        char             mf_padding[15];
    } invt_mediafile_t;

The data structures above converted to a block diagram look something like this:

The source code for accessing the inventory is contained in the inventory directory. The source code for xsfinvutil is contained in the invutil directory. xfsinvutil only uses some header files from the inventory directory for data structure definitions -- it uses its own code to access and modify the inventory.

### Questions and Answers

How is -a and -z handled by xfsdump ?
  If -a is NOT used then it looks like nothing special happens for files which have dmf state attached to them. So if the file uses too many blocks compared to our maxsize param (-z) then it will not get dumped. No inode nor data. The only evidence will be its entry in the inode map (which is dumped) which says its the state of a no-change-non-dir and the directory entry in the directories dump. The latter will mean that an ls in xfsrestore will show the file but it can not be restored.

  If -a is used and the file has some DMF state then we do some magic. However, the magic really only seems to occur for dual-state files (or possibly also unmigrating files).

  A file is marked as dual-state/unmigrating by looking at the DMF attribute, dmfattrp->state[1]. i.e = DMF_ST_DUALSTATE or DMF_ST_UNMIGRATING If this is the case, then we set, dmf_f_ctxtp->candidate = 1. If we have such a changed dual-state file then we mark it as changed in the inode-map so it can be dumped. If it is a dual state file, then its apparent size will be zero, so it will go onto the dumping stage.

  When we go to dump the extents of the dual-state file, we do something different. We store the extents as only 1 extent which is a hole. I.e. this is the "NOT dumping data" bit.

  When we go to dump the file-hdr of the dual-state file, we set, statp->bs_dmevmask |= (1< When we go to dump the extended-attributes of the dual-state file, we skip dumping the DMF attribute ones ! However, at the end of dumping the attributes, we then go and add a new DMF attribute for it:

              dmfattrp->state[1] = DMF_ST_OFFLINE;
              *valuepp = (char *)dmfattrp;
              *namepp = DMF_ATTR_NAME;
              *valueszp = DMF_ATTR_LEN;

  Summary:
  - dual state files (and unmigrating files) dumped with -a, cause magic to happen:
    - if file has changed then it will _always_ be marked to be dumped out (irrespective of file size/blocks)
    - its extent data will be dumped as 1 extent with a hole
    - its DMF attributes won't be dumped but a replacement DMF attribute will be dumped in its place
    - the stat buf's bs_devmask will be or'ed with DM_EVENT_READ
  - for all other cases, if the file has changed and its blocks cause it to exceed the maxsize param (-z) then the file will be marked as NOT-CHANGED in the inode map and so will NOT be dumped at all

How does it compute estimated dump size ?
  A dump consists of media files (only 1 in the case of a dump to a file, and usually many when dumped to a tape (depending on device type)). A media file consists of:
  - global header
  - inode map (inode# + state(e.g.dump or not?) )
  - directories
  - non-directory files

  A directory consists of a header, directory-entry-headers for its entries and extended-attribute header and attributes.

  A non-directory file consists of a file header, extent-headers (for each extent), file data and extended-attribute header and attributes. Some types of files don't have extent headers or data.

  The xfsdump code says:

              size_estimate = GLOBAL_HDR_SZ
                              +
                              inomap_getsz( )
                              +
                              inocnt * ( u_int64_t )( FILEHDR_SZ + EXTENTHDR_SZ )
                              +
                              inocnt * ( u_int64_t )( DIRENTHDR_SZ + 8 )
                              +
                              datasz;

  So this accounts for the:
  - global header
  - inode map
  - all the files
  - all the direntory entries ( "+8" presumably to account for average file name length range, where 8 chars already included in header; as this structure is padded to the next 8 byte boundary, it accounts for names with lengths between 8-15 chars)
  - data

  What estimate doesn't seem to account for (that I can think of):

  - no extended attributes
  - assumes that a file will only have one extent
  - no tape block headers (for tape media)

  "Datasz" is calculated by adding up for every regular inode file, its (number of data blocks) * (block size). However, if "-a" is used, then instead of doing this, if the file is dualstate/offline then the file's data won't be dumped and it adds zero for it.

Is the "dump size (non-dir files) : 910617928 bytes" the actual number of bytes it wrote to that tape ?
  It is the number of bytes it wrote to the dump for the non-directory files' extents (not including file header nor extent header terminator). (I don't think this includes the tape block headers for a tape dump either.) It includes for each file:
  - any hole hdrs
  - alignment hdrs
  - alignment padding
  - extent headers for data
  - actual _data_ of extents
  From code:

          bytecnt += sizeof( filehdr_t );
          dump_extent_group(...,&bc,...);
      	bytecnt = 0;
      	bytecnt += sizeof( extenthdr_t );  /* extent header for hole */
      	bytecnt += sizeof( extenthdr_t );  /* ext. alignment header */
      	bytecnt += ( off64_t )cnt_to_align /* alignment padding */
      	bytecnt += sizeof( extenthdr_t );  /* extent header for data */
      	bytecnt += ( off64_t )actualsz;    /* actual extent data in file */
      	bytecnt += ( off64_t )reqsz; /* write padding to make up extent size */
          sc_stat_datadone += ( size64_t )bc;

  It doesn't include the initial file header:

          rv = dump_filehdr( ... );
          bytecnt += sizeof( filehdr_t );

  nor the extent hdr terminator:

          rv = dump_extenthdr( ..., EXTENTHDR_TYPE_LAST,...);
          bytecnt += sizeof( extenthdr_t );
          contextp->cc_mfilesz += bytecnt;

  It only adds this data size into the media file size.

### Outstanding Questions

- How is the inode map on the tape used by xfsrestore ?
- Is the final inventory media file on the media ever used/restored ?
- How are tape marks used and written ?
- What is the difference between a record and a block ?
  - I don't think there is a difference.
- Where are tape_recsz and tape_blksz used ?
  - Tape_recsz is used for the read/write byte cnt but I don't think tape_blksz is used.
- What is the persistent inventory used for ?