An XFS filesystem can reside on a regular disk partition or on a
An XFS filesystem has up to three parts:
a data section, a log section, and a realtime section.
Using the default
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
time with the
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
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
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
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
It is written sequentially during normal operation and read only
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
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
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
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
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
or other block-by-block copying programs to copy XFS filesystems.
If two XFS filesystems on the same machine have the same UUID,
may become confused when doing incremental and resumed dumps.
are recommended for making copies of XFS filesystems.