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 sec-
tion 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, directo-
ries, 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 sec-
tion. 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 sequen-
tially during normal operation and read only during mount. When mount-
ing 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 di-
vided 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 sec-
tor 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 ap-
plications 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 values for this option are
page size (typically 4KiB) through to 1GiB, inclusive, in power-
of-2 increments.
The default behavior is for dynamic end-of-file preallocation
size, which uses a set of heuristics to optimise the prealloca-
tion size based on the current allocation patterns within the
file and the access patterns to the file. Specifying a fixed al-
locsize value turns off the dynamic behavior.
attr2|noattr2
Note: These options have been deprecated as of kernel v5.10; The
noattr2 option will be removed no earlier than in September 2025
and attr2 option will be immutable default.
The options enable/disable an "opportunistic" improvement to be
made in the way inline extended attributes are stored on-disk.
When the new form is used for the first time when attr2 is se-
lected (either when setting or removing extended attributes) the
on-disk superblock feature bit field will be updated to reflect
this format being in use.
The default behavior is determined by the on-disk feature bit in-
dicating that attr2 behavior is active. If either mount option it
set, then that becomes the new default used by the filesystem.
CRC enabled filesystems always use the attr2 format, and so will
reject the noattr2 mount option if it is set.
dax=value
Set CPU direct access (DAX) behavior for the current filesystem.
This mount option accepts the following values:
"dax=inode" DAX will be enabled only on regular files with
FS_XFLAG_DAX applied.
"dax=never" DAX will not be enabled for any files. FS_XFLAG_DAX
will be ignored.
"dax=always" DAX will be enabled for all regular files, regard-
less of the FS_XFLAG_DAX state.
If no option is used when mounting a filesystem stored on a DAX
capable device, dax=inode will be used as default.
For details regarding DAX behavior in kernel, please refer to
kernel's documentation at filesystems/dax.txt
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 performance impact.
Note: It is currently recommended that you use the fstrim appli-
cation to discard unused blocks rather than the discard mount op-
tion because the performance impact of this option is quite se-
vere. 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 con-
figured to use it.
ikeep|noikeep
Note: These options have been deprecated as of kernel v5.10; The
noikeep option will be removed no earlier than in September 2025
and ikeep option will be immutable default.
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
significance.
inode32 is provided for backwards compatibility with older sys-
tems 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 speci-
fied.
For kernel v3.7 and later, inode64 is the default.
largeio|nolargeio
If "nolargeio" is specified, the optimal I/O reported in st_blk-
size by stat(2) will be as small as possible to allow user appli-
cations to avoid inefficient read/modify/write I/O. This is typ-
ically the page size of the machine, as this is the granularity
of the page cache.
If "largeio" specified, a filesystem that was created with a
"swidth" specified will return the "swidth" value (in bytes) in
st_blksize. If the filesystem does not have a "swidth" specified
but does specify an "allocsize" then "allocsize" (in bytes) will
be returned instead. Otherwise the behavior is the same as if
"nolargeio" was specified. nolargeio is the default.
logbufs=value
Set the number of in-memory log buffers. Valid numbers range
from 2–8 inclusive.
The default value is 8 buffers.
If the memory cost of 8 log buffers is too high on small systems,
then it may be reduced at some cost to performance on metadata
intensive workloads. The logbsize option below controls the size
of each buffer and so is also relevant to this case.
logbsize=value
Set the size of each in-memory log buffer. The size may be spec-
ified 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 op-
tional, 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 in-
consistent when mounted in "norecovery" mode. Some files or di-
rectories 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) en-
forced. Refer to xfs_quota(8) for further details.
gquota/grpquota/gqnoenforce
Group disk quota accounting enabled and limits (optionally) en-
forced. Refer to xfs_quota(8) for further details.
pquota/prjquota/pqnoenforce
Project disk quota accounting enabled and limits (optionally) en-
forced. Refer to xfs_quota(8) for further 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 specified 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.
Typically the only time these mount options are necessary if af-
ter an underlying RAID device has had it's geometry modified,
such as adding a new disk to a RAID5 lun and reshaping it.
swalloc
Data allocations will be rounded up to stripe width boundaries
when the current end of file is being extended and the file size
is larger than the stripe width size.
wsync When specified, all filesystem namespace operations are executed
synchronously. This ensures that when the namespace operation
(create, unlink, etc) completes, the change to the namespace is
on stable storage. This is useful in HA setups where failover
must not result in clients seeing inconsistent namespace presen-
tation during or after a failover event.
REMOVED MOUNT OPTIONS
The following mount options have been removed from the kernel, and will
yield mount failures if specified. Mount options are deprecated for a
significant period time prior to removal.
Name Removed
---- -------
delaylog/nodelaylog v4.0
ihashsize v4.0
irixsgid v4.0
osyncisdsync/osyncisosync v4.0
barrier/nobarrier v4.19
FILE ATTRIBUTES
The XFS filesystem supports setting the following file attributes on
Linux systems using the chattr(1) utility:
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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