.TH xfs 5
.SH NAME
xfs \- layout of the XFS filesystem
.SH DESCRIPTION
An XFS filesystem can reside on a regular disk partition or on a
logical volume (see
.IR lvm (8)).
An XFS filesystem has up to three parts:
a data section, a log section, and a real-time section.
For disk partition filesystems,
the real-time section is absent, and
the log area is contained within the data section.
For logical volume filesystems,
the real-time section is optional,
and the log section can be separate from the data section
or contained within it.
The filesystem sections are divided into a certain number of
.IR blocks ,
whose size is specified at
.IR mkfs (8)
time with the
.B \-b
option.
.PP
The data section contains all the filesystem metadata
(inodes, directories, indirect blocks)
as well as the user file data for ordinary (non-real-time) files
and the log area if the log is
.I internal
to the data section.
The data section is divided into a number of
\f2allocation groups\f1.
The number and size of the allocation groups are chosen by
.I mkfs
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
.IR xfs_growfs (8)
is run.
.PP
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.
.PP
The real-time section is used to store the data of real-time files.
These files had an attribute bit set through
.IR fcntl (2)
after file creation, before any data was written to the file.
The real-time section is divided into a number of
.I extents
of fixed size (specified at
.I mkfs
time).
Each file in the real-time section has an extent size that
is a multiple of the real-time section extent size.
.PP
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
.IR mkfs .
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.
.PP
Each XFS filesystem is labeled with a unique
universal 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
.I dd
or other block-by-block copying programs to copy XFS filesystems.
If two XFS filesystems on the same machine have the UUID,
.I xfsdump
may become confused when doing incremental and resumed dumps.
(See
.IR xfsdump (8)
for more details.)
.I xfs_copy
or
.IR xfsdump / xfsrestore
are recommended for making copies of XFS filesystems.
.PP
All these data structures are subject to change, and the
headers that specify their layout on disk are not provided.
.SH SEE ALSO
fs(5),
mkfs.xfs(8),
xfs_bmap(8),
xfs_check(8),
xfs_copy(8),
xfs_estimate(8),
xfs_growfs(8),
xfs_logprint(8),
xfs_repair(8),
xfsdump(8),
xfsrestore(8),
fcntl(2),
lvm(8).