Provided by: xfsprogs_4.9.0+nmu1ubuntu2_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)