Section: File Formats (5)
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xfs - layout, mount options, and supported file attributes for the XFS filesystem
An XFS filesystem can reside on a regular disk partition or on a
An XFS filesystem has up to three parts:
a data section, a log section, and a realtime section.
Using the default
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
whose size is specified at
time with the
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
to the data section.
The data section is divided into a number of
The number and size of the allocation groups are chosen by
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
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
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
It is written sequentially during normal operation and read only
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
after file creation, before any data was written to the file.
The realtime section is divided into a number of
of fixed size (specified at
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
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
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
or other block-by-block copying programs to copy XFS filesystems.
If two XFS filesystems on the same machine have the same UUID,
may become confused when doing incremental and resumed dumps.
are recommended for making copies of XFS filesystems.
Some functionality specific to the XFS filesystem is accessible to
applications through the
and by-handle (see
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
manual page for more details.
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.
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.
Note: This option has been
as of kernel v4.10; in that version, integrity operations are
always performed and the mount option is ignored. These mount
options will be removed no earlier than kernel v4.15.
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.
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.
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.
Make the data allocator use the filestreams allocation mode
across the entire filesystem rather than just on directories
configured to use it.
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.
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.
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.
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
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.
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
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.
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
Forcibly turns off all quota accounting and enforcement
within the filesystem.
User disk quota accounting enabled, and limits (optionally)
enforced. Refer to xfs_quota(8) for further details.
Group disk quota accounting enabled and limits (optionally)
enforced. Refer to xfs_quota(8) for further details.
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
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.
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
The XFS filesystem supports setting the following file
attributes on Linux systems using the
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