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xfs(5)                    FreeBSD File Formats Manual                   xfs(5)

       xfs - layout of the XFS filesystem

       An XFS filesystem can reside on a regular disk partition or on a
       logical volume.  An XFS filesystem has up to three parts: a data
       section, a log section, and a realtime section.  Using the default
       mkfs.xfs(8) options, the realtime section is absent, and the log area
       is contained within the data section.  The log section can be either
       separate from the data section or contained within it.  The filesystem
       sections are divided into a certain number of blocks, whose size is
       specified at mkfs.xfs(8) time with the -b option.

       The data section contains all the filesystem metadata (inodes,
       directories, indirect blocks) as well as the user file data for
       ordinary (non-realtime) files and the log area if the log is internal
       to the data section.  The data section is divided into a number of
       allocation groups.  The number and size of the allocation groups are
       chosen by mkfs.xfs(8) so that there is normally a small number of
       equal-sized groups.  The number of allocation groups controls the
       amount of parallelism available in file and block allocation.  It
       should be increased from the default if there is sufficient memory and
       a lot of allocation activity.  The number of allocation groups should
       not be set very high, since this can cause large amounts of CPU time to
       be used by the filesystem, especially when the filesystem is nearly
       full.  More allocation groups are added (of the original size) when
       xfs_growfs(8) is run.

       The log section (or area, if it is internal to the data section) is
       used to store changes to filesystem metadata while the filesystem is
       running until those changes are made to the data section.  It is
       written sequentially during normal operation and read only during
       mount.  When mounting a filesystem after a crash, the log is read to
       complete operations that were in progress at the time of the crash.

       The realtime section is used to store the data of realtime files.
       These files had an attribute bit set through xfsctl(3) after file
       creation, before any data was written to the file.  The realtime
       section is divided into a number of extents of fixed size (specified at
       mkfs.xfs(8) time).  Each file in the realtime section has an extent
       size that is a multiple of the realtime section extent size.

       Each allocation group contains several data structures.  The first
       sector contains the superblock.  For allocation groups after the first,
       the superblock is just a copy and is not updated after mkfs.xfs(8).
       The next three sectors contain information for block and inode
       allocation within the allocation group.  Also contained within each
       allocation group are data structures to locate free blocks and inodes;
       these are located through the header structures.

       Each XFS filesystem is labeled with a Universal Unique Identifier
       (UUID).  The UUID is stored in every allocation group header and is
       used to help distinguish one XFS filesystem from another, therefore you
       should avoid using dd(1) or other block-by-block copying programs to
       copy XFS filesystems.  If two XFS filesystems on the same machine have
       the same UUID, xfsdump(8) may become confused when doing incremental
       and resumed dumps.  xfsdump(8) and xfsrestore(8) are recommended for
       making copies of XFS filesystems.

       Some functionality specific to the XFS filesystem is accessible to
       applications through the xfsctl(3) and by-handle (see
       open_by_handle(3)) interfaces.

       Refer to the mount(8) manual entry for descriptions of the individual
       XFS mount options.

       xfsctl(3), mount(8), mkfs.xfs(8), xfs_info(8), xfs_admin(8),
       xfsdump(8), xfsrestore(8).



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