xfs(5) File Formats Manual xfs(5)
NAME
xfs - layout, mount options, and supported file attributes for 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 cer‐
tain 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 allo‐
cation 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 orig‐
inal 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 meta‐
data while the filesystem is running until those changes are made to the data section. It is written sequen‐
tially 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 allo‐
cation 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 alloca‐
tion 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
The following XFS-specific mount options may be used when mounting an XFS filesystem. Other generic options
may be used as well; refer to the mount(8) manual page for more details.
allocsize=size
Sets the buffered I/O end-of-file preallocation size when doing delayed allocation writeout. Valid val‐
ues for this option are page size (typically 4KiB) through to 1GiB, inclusive, in power-of-2 incre‐
ments.
The default behavior is for dynamic end-of-file preallocation size, which uses a set of heuristics to
CRC enabled filesystems always use the attr2 format, and so will reject the noattr2 mount option if it
is set.
barrier|nobarrier
Enables/disables the use of block layer write barriers for writes into the journal and for data
integrity operations. This allows for drive level write caching to be enabled, for devices that sup‐
port write barriers.
Barriers are enabled by default.
discard|nodiscard
Enable/disable the issuing of commands to let the block device reclaim space freed by the filesystem.
This is useful for SSD devices, thinly provisioned LUNs and virtual machine images, but may have a per‐
formance impact.
Note: It is currently recommended that you use the fstrim application to discard unused blocks rather
than the discard mount option because the performance impact of this option is quite severe. For this
reason, nodiscard is the default.
grpid|bsdgroups|nogrpid|sysvgroups
These options define what group ID a newly created file gets. When grpid is set, it takes the group ID
of the directory in which it is created; otherwise it takes the fsgid of the current process, unless
the directory has the setgid bit set, in which case it takes the gid from the parent directory, and
also gets the setgid bit set if it is a directory itself.
filestreams
Make the data allocator use the filestreams allocation mode across the entire filesystem rather than
just on directories configured to use it.
ikeep|noikeep
When ikeep is specified, XFS does not delete empty inode clusters and keeps them around on disk. When
noikeep is specified, empty inode clusters are returned to the free space pool. noikeep is the
default.
inode32|inode64
When inode32 is specified, it indicates that XFS limits inode creation to locations which will not
result in inode numbers with more than 32 bits of significance.
When inode64 is specified, it indicates that XFS is allowed to create inodes at any location in the
filesystem, including those which will result in inode numbers occupying more than 32 bits of signifi‐
cance.
inode32 is provided for backwards compatibility with older systems and applications, since 64 bits
inode numbers might cause problems for some applications that cannot handle large inode numbers. If
applications are in use which do not handle inode numbers bigger than 32 bits, the inode32 option
should be specified.
For kernel v3.7 and later, inode64 is the default.
largeio|nolargeio
If "nolargeio" is specified, the optimal I/O reported in st_blksize by stat(2) will be as small as pos‐
sible to allow user applications to avoid inefficient read/modify/write I/O. This is typically the
page size of the machine, as this is the granularity of the page cache.
buffer and so is also relevant to this case.
logbsize=value
Set the size of each in-memory log buffer. The size may be specified in bytes, or in kibibytes (KiB)
with a "k" suffix. Valid sizes for version 1 and version 2 logs are 16384 (value=16k) and 32768
(value=32k). Valid sizes for version 2 logs also include 65536 (value=64k), 131072 (value=128k) and
262144 (value=256k). The logbsize must be an integer multiple of the log stripe unit configured at mkfs
time.
The default value for version 1 logs is 32768, while the default value for version 2 logs is MAX(32768,
log_sunit).
logdev=device and rtdev=device
Use an external log (metadata journal) and/or real-time device. An XFS filesystem has up to three
parts: a data section, a log section, and a real-time section. The real-time section is optional, and
the log section can be separate from the data section or contained within it.
noalign
Data allocations will not be aligned at stripe unit boundaries. This is only relevant to filesystems
created with non-zero data alignment parameters (sunit, swidth) by mkfs.
norecovery
The filesystem will be mounted without running log recovery. If the filesystem was not cleanly
unmounted, it is likely to be inconsistent when mounted in "norecovery" mode. Some files or directo‐
ries may not be accessible because of this. Filesystems mounted "norecovery" must be mounted read-only
or the mount will fail.
nouuid Don't check for double mounted file systems using the file system uuid. This is useful to mount LVM
snapshot volumes, and often used in combination with "norecovery" for mounting read-only snapshots.
noquota
Forcibly turns off all quota accounting and enforcement within the filesystem.
uquota/usrquota/quota/uqnoenforce/qnoenforce
User disk quota accounting enabled, and limits (optionally) enforced. Refer to xfs_quota(8) for fur‐
ther details.
gquota/grpquota/gqnoenforce
Group disk quota accounting enabled and limits (optionally) enforced. Refer to xfs_quota(8) for fur‐
ther details.
pquota/prjquota/pqnoenforce
Project disk quota accounting enabled and limits (optionally) enforced. Refer to xfs_quota(8) for fur‐
ther details.
sunit=value and swidth=value
Used to specify the stripe unit and width for a RAID device or a stripe volume. "value" must be speci‐
fied in 512-byte block units. These options are only relevant to filesystems that were created with
non-zero data alignment parameters.
The sunit and swidth parameters specified must be compatible with the existing filesystem alignment
characteristics. In general, that means the only valid changes to sunit are increasing it by a power-
of-2 multiple. Valid swidth values are any integer multiple of a valid sunit value.
FILE ATTRIBUTES
The XFS filesystem supports setting the following file attributes on Linux systems using the chattr(1) util‐
ity:
a - append only
A - no atime updates
d - no dump
i - immutable
S - synchronous updates
For descriptions of these attribute flags, please refer to the chattr(1) man page.
SEE ALSO
chattr(1), xfsctl(3), mount(8), mkfs.xfs(8), xfs_info(8), xfs_admin(8), xfsdump(8), xfsrestore(8).
xfs(5)
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