Provided by: xfsprogs_4.3.0+nmu1ubuntu1.1_amd64 
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 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, 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 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(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
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 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
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 preallocation size based on the current allocation patterns within the file and
the access patterns to the file. Specifying a fixed allocsize value turns off the dynamic
behavior.
attr2|noattr2
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
selected (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 indicating 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.
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
support 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 performance 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
significance.
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
possible 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.
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 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
directories 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
further details.
gquota/grpquota/gqnoenforce
Group disk quota accounting enabled and limits (optionally) enforced. Refer to xfs_quota(8) for
further details.
pquota/prjquota/pqnoenforce
Project disk quota accounting enabled and limits (optionally) enforced. 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 after 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 presentation during or after a failover event.
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)