xfs(5)								File Formats Manual							    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 section is absent, and the  log  area  is  con-
       tained  within the data section.  The log section can be either separate from the data section or contained within it.  The filesystem sec-
       tions 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  cre-
       ation,  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 alloca-
       tion 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)