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 logi-
cal 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, 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
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 paral-
lelism 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 allo-
cation 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 writ-
ten 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 cre-
ation, 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 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 alloca-
tion within the allocation group. Also contained within each alloca-
tion 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_han-
dle(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
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 filesys-
tem.
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 appli-
cation 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 clus-
ters 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
applications to avoid inefficient read/modify/write I/O. This
is typically the page size of the machine, as this is the granu-
larity 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 sys-
tems, 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 sec-
tion 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 sys-
tem 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 gen-
eral, 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 presen-
tation 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)
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