xfs - layout of the XFS filesystem
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.
Some functionality specific to the XFS filesystem is accessible to
applications through the xfsctl(3) and by-handle (see
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),