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xfs(5)									xfs(5)

       xfs - layout of the XFS filesystem

       An  XFS filesystem can reside on	a regular disk partition or on a logi-
       cal 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, directo-
       ries, 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	paral-
       lelism  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 allo-
       cation groups are added (of the original	size)  when  xfs_growfs(8)  is

       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 writ-
       ten 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 sec-
       tor 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 alloca-
       tion 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_han-
       dle(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),  xfs-
       dump(8),	xfsrestore(8).



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