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 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
<2>allocation groups<1>.
The number and size of the allocation groups are chosen by
mkfs.xfs 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 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 . 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 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 and
xfsrestore 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.