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 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 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.

Some functionality specific to the XFS filesystem is accessible to applications through the xfsctl(3) and by-handle (see open_by_handle(3)) interfaces.

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), xfsdump(8), xfsrestore(8).