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 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.

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)