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).