xfs(5)                                                                                                                    xfs(5)



NAME
       xfs - layout of the XFS filesystem

DESCRIPTION
       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  sec-
       tion  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 par-
       allelism  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.

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

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

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



                                                                                                                          xfs(5)