.. SPDX-License-Identifier: GPL-2.0

======================
The SGI XFS Filesystem
======================

XFS is a high performance journaling filesystem which originated
on the SGI IRIX platform.  It is completely multi-threaded, can
support large files and large filesystems, extended attributes,
variable block sizes, is extent based, and makes extensive use of
Btrees (directories, extents, free space) to aid both performance
and scalability.

Refer to the documentation at https://xfs.wiki.kernel.org/
for further details.  This implementation is on-disk compatible
with the IRIX version of XFS.


Mount Options
=============

When mounting an XFS filesystem, the following options are accepted.

  allocsize=size
	Sets the buffered I/O end-of-file preallocation size when
	doing delayed allocation writeout (default size is 64KiB).
	Valid values for this option are page size (typically 4KiB)
	through to 1GiB, inclusive, in power-of-2 increments.

	The default behaviour 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 behaviour.

  attr2 or 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 behaviour is determined by the on-disk feature
	bit indicating that ``attr2`` behaviour is active. If either
	mount option is 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.

  discard or nodiscard (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.

  grpid/bsdgroups or nogrpid/sysvgroups (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.

  filestreams
	Make the data allocator use the filestreams allocation mode
	across the entire filesystem rather than just on directories
	configured to use it.

  ikeep or noikeep (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.

  inode32 or inode64 (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.

  largeio or nolargeio (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`` is 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 behaviour
	is the same as if ``nolargeio`` was specified.

  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 kilobytes with a "k" suffix.
	Valid sizes for version 1 and version 2 logs are 16384 (16k)
	and 32768 (32k).  Valid sizes for version 2 logs also
	include 65536 (64k), 131072 (128k) and 262144 (256k). The
	logbsize must be an integer multiple of the log
	stripe unit configured at **mkfs(8)** 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(8)**.

  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/uqnoenforce/quota
	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.

Deprecation of V4 Format
========================

The V4 filesystem format lacks certain features that are supported by
the V5 format, such as metadata checksumming, strengthened metadata
verification, and the ability to store timestamps past the year 2038.
Because of this, the V4 format is deprecated.  All users should upgrade
by backing up their files, reformatting, and restoring from the backup.

Administrators and users can detect a V4 filesystem by running xfs_info
against a filesystem mountpoint and checking for a string containing
"crc=".  If no such string is found, please upgrade xfsprogs to the
latest version and try again.

The deprecation will take place in two parts.  Support for mounting V4
filesystems can now be disabled at kernel build time via Kconfig option.
The option will default to yes until September 2025, at which time it
will be changed to default to no.  In September 2030, support will be
removed from the codebase entirely.

Note: Distributors may choose to withdraw V4 format support earlier than
the dates listed above.

Deprecated Mount Options
========================

===========================     ================
  Name				Removal Schedule
===========================     ================
Mounting with V4 filesystem     September 2030
ikeep/noikeep			September 2025
attr2/noattr2			September 2025
===========================     ================


Removed Mount Options
=====================

===========================     =======
  Name				Removed
===========================	=======
  delaylog/nodelaylog		v4.0
  ihashsize			v4.0
  irixsgid			v4.0
  osyncisdsync/osyncisosync	v4.0
  barrier			v4.19
  nobarrier			v4.19
===========================     =======

sysctls
=======

The following sysctls are available for the XFS filesystem:

  fs.xfs.stats_clear		(Min: 0  Default: 0  Max: 1)
	Setting this to "1" clears accumulated XFS statistics
	in /proc/fs/xfs/stat.  It then immediately resets to "0".

  fs.xfs.xfssyncd_centisecs	(Min: 100  Default: 3000  Max: 720000)
	The interval at which the filesystem flushes metadata
	out to disk and runs internal cache cleanup routines.

  fs.xfs.filestream_centisecs	(Min: 1  Default: 3000  Max: 360000)
	The interval at which the filesystem ages filestreams cache
	references and returns timed-out AGs back to the free stream
	pool.

  fs.xfs.speculative_prealloc_lifetime
	(Units: seconds   Min: 1  Default: 300  Max: 86400)
	The interval at which the background scanning for inodes
	with unused speculative preallocation runs. The scan
	removes unused preallocation from clean inodes and releases
	the unused space back to the free pool.

  fs.xfs.speculative_cow_prealloc_lifetime
	This is an alias for speculative_prealloc_lifetime.

  fs.xfs.error_level		(Min: 0  Default: 3  Max: 11)
	A volume knob for error reporting when internal errors occur.
	This will generate detailed messages & backtraces for filesystem
	shutdowns, for example.  Current threshold values are:

		XFS_ERRLEVEL_OFF:       0
		XFS_ERRLEVEL_LOW:       1
		XFS_ERRLEVEL_HIGH:      5

  fs.xfs.panic_mask		(Min: 0  Default: 0  Max: 256)
	Causes certain error conditions to call BUG(). Value is a bitmask;
	OR together the tags which represent errors which should cause panics:

		XFS_NO_PTAG                     0
		XFS_PTAG_IFLUSH                 0x00000001
		XFS_PTAG_LOGRES                 0x00000002
		XFS_PTAG_AILDELETE              0x00000004
		XFS_PTAG_ERROR_REPORT           0x00000008
		XFS_PTAG_SHUTDOWN_CORRUPT       0x00000010
		XFS_PTAG_SHUTDOWN_IOERROR       0x00000020
		XFS_PTAG_SHUTDOWN_LOGERROR      0x00000040
		XFS_PTAG_FSBLOCK_ZERO           0x00000080
		XFS_PTAG_VERIFIER_ERROR         0x00000100

	This option is intended for debugging only.

  fs.xfs.irix_symlink_mode	(Min: 0  Default: 0  Max: 1)
	Controls whether symlinks are created with mode 0777 (default)
	or whether their mode is affected by the umask (irix mode).

  fs.xfs.irix_sgid_inherit	(Min: 0  Default: 0  Max: 1)
	Controls files created in SGID directories.
	If the group ID of the new file does not match the effective group
	ID or one of the supplementary group IDs of the parent dir, the
	ISGID bit is cleared if the irix_sgid_inherit compatibility sysctl
	is set.

  fs.xfs.inherit_sync		(Min: 0  Default: 1  Max: 1)
	Setting this to "1" will cause the "sync" flag set
	by the **xfs_io(8)** chattr command on a directory to be
	inherited by files in that directory.

  fs.xfs.inherit_nodump		(Min: 0  Default: 1  Max: 1)
	Setting this to "1" will cause the "nodump" flag set
	by the **xfs_io(8)** chattr command on a directory to be
	inherited by files in that directory.

  fs.xfs.inherit_noatime	(Min: 0  Default: 1  Max: 1)
	Setting this to "1" will cause the "noatime" flag set
	by the **xfs_io(8)** chattr command on a directory to be
	inherited by files in that directory.

  fs.xfs.inherit_nosymlinks	(Min: 0  Default: 1  Max: 1)
	Setting this to "1" will cause the "nosymlinks" flag set
	by the **xfs_io(8)** chattr command on a directory to be
	inherited by files in that directory.

  fs.xfs.inherit_nodefrag	(Min: 0  Default: 1  Max: 1)
	Setting this to "1" will cause the "nodefrag" flag set
	by the **xfs_io(8)** chattr command on a directory to be
	inherited by files in that directory.

  fs.xfs.rotorstep		(Min: 1  Default: 1  Max: 256)
	In "inode32" allocation mode, this option determines how many
	files the allocator attempts to allocate in the same allocation
	group before moving to the next allocation group.  The intent
	is to control the rate at which the allocator moves between
	allocation groups when allocating extents for new files.

Deprecated Sysctls
==================

===========================================     ================
  Name                                          Removal Schedule
===========================================     ================
fs.xfs.irix_sgid_inherit                        September 2025
fs.xfs.irix_symlink_mode                        September 2025
fs.xfs.speculative_cow_prealloc_lifetime        September 2025
===========================================     ================


Removed Sysctls
===============

=============================	=======
  Name				Removed
=============================	=======
  fs.xfs.xfsbufd_centisec	v4.0
  fs.xfs.age_buffer_centisecs	v4.0
=============================	=======

Error handling
==============

XFS can act differently according to the type of error found during its
operation. The implementation introduces the following concepts to the error
handler:

 -failure speed:
	Defines how fast XFS should propagate an error upwards when a specific
	error is found during the filesystem operation. It can propagate
	immediately, after a defined number of retries, after a set time period,
	or simply retry forever.

 -error classes:
	Specifies the subsystem the error configuration will apply to, such as
	metadata IO or memory allocation. Different subsystems will have
	different error handlers for which behaviour can be configured.

 -error handlers:
	Defines the behavior for a specific error.

The filesystem behavior during an error can be set via ``sysfs`` files. Each
error handler works independently - the first condition met by an error handler
for a specific class will cause the error to be propagated rather than reset and
retried.

The action taken by the filesystem when the error is propagated is context
dependent - it may cause a shut down in the case of an unrecoverable error,
it may be reported back to userspace, or it may even be ignored because
there's nothing useful we can with the error or anyone we can report it to (e.g.
during unmount).

The configuration files are organized into the following hierarchy for each
mounted filesystem:

  /sys/fs/xfs/<dev>/error/<class>/<error>/

Where:
  <dev>
	The short device name of the mounted filesystem. This is the same device
	name that shows up in XFS kernel error messages as "XFS(<dev>): ..."

  <class>
	The subsystem the error configuration belongs to. As of 4.9, the defined
	classes are:

		- "metadata": applies metadata buffer write IO

  <error>
	The individual error handler configurations.


Each filesystem has "global" error configuration options defined in their top
level directory:

  /sys/fs/xfs/<dev>/error/

  fail_at_unmount		(Min:  0  Default:  1  Max: 1)
	Defines the filesystem error behavior at unmount time.

	If set to a value of 1, XFS will override all other error configurations
	during unmount and replace them with "immediate fail" characteristics.
	i.e. no retries, no retry timeout. This will always allow unmount to
	succeed when there are persistent errors present.

	If set to 0, the configured retry behaviour will continue until all
	retries and/or timeouts have been exhausted. This will delay unmount
	completion when there are persistent errors, and it may prevent the
	filesystem from ever unmounting fully in the case of "retry forever"
	handler configurations.

	Note: there is no guarantee that fail_at_unmount can be set while an
	unmount is in progress. It is possible that the ``sysfs`` entries are
	removed by the unmounting filesystem before a "retry forever" error
	handler configuration causes unmount to hang, and hence the filesystem
	must be configured appropriately before unmount begins to prevent
	unmount hangs.

Each filesystem has specific error class handlers that define the error
propagation behaviour for specific errors. There is also a "default" error
handler defined, which defines the behaviour for all errors that don't have
specific handlers defined. Where multiple retry constraints are configured for
a single error, the first retry configuration that expires will cause the error
to be propagated. The handler configurations are found in the directory:

  /sys/fs/xfs/<dev>/error/<class>/<error>/

  max_retries			(Min: -1  Default: Varies  Max: INTMAX)
	Defines the allowed number of retries of a specific error before
	the filesystem will propagate the error. The retry count for a given
	error context (e.g. a specific metadata buffer) is reset every time
	there is a successful completion of the operation.

	Setting the value to "-1" will cause XFS to retry forever for this
	specific error.

	Setting the value to "0" will cause XFS to fail immediately when the
	specific error is reported.

	Setting the value to "N" (where 0 < N < Max) will make XFS retry the
	operation "N" times before propagating the error.

  retry_timeout_seconds		(Min:  -1  Default:  Varies  Max: 1 day)
	Define the amount of time (in seconds) that the filesystem is
	allowed to retry its operations when the specific error is
	found.

	Setting the value to "-1" will allow XFS to retry forever for this
	specific error.

	Setting the value to "0" will cause XFS to fail immediately when the
	specific error is reported.

	Setting the value to "N" (where 0 < N < Max) will allow XFS to retry the
	operation for up to "N" seconds before propagating the error.

**Note:** The default behaviour for a specific error handler is dependent on both
the class and error context. For example, the default values for
"metadata/ENODEV" are "0" rather than "-1" so that this error handler defaults
to "fail immediately" behaviour. This is done because ENODEV is a fatal,
unrecoverable error no matter how many times the metadata IO is retried.

Workqueue Concurrency
=====================

XFS uses kernel workqueues to parallelize metadata update processes.  This
enables it to take advantage of storage hardware that can service many IO
operations simultaneously.  This interface exposes internal implementation
details of XFS, and as such is explicitly not part of any userspace API/ABI
guarantee the kernel may give userspace.  These are undocumented features of
the generic workqueue implementation XFS uses for concurrency, and they are
provided here purely for diagnostic and tuning purposes and may change at any
time in the future.

The control knobs for a filesystem's workqueues are organized by task at hand
and the short name of the data device.  They all can be found in:

  /sys/bus/workqueue/devices/${task}!${device}

================  ===========
  Task            Description
================  ===========
  xfs_iwalk-$pid  Inode scans of the entire filesystem. Currently limited to
                  mount time quotacheck.
  xfs-gc          Background garbage collection of disk space that have been
                  speculatively allocated beyond EOF or for staging copy on
                  write operations.
================  ===========

For example, the knobs for the quotacheck workqueue for /dev/nvme0n1 would be
found in /sys/bus/workqueue/devices/xfs_iwalk-1111!nvme0n1/.

The interesting knobs for XFS workqueues are as follows:

============     ===========
  Knob           Description
============     ===========
  max_active     Maximum number of background threads that can be started to
                 run the work.
  cpumask        CPUs upon which the threads are allowed to run.
  nice           Relative priority of scheduling the threads.  These are the
                 same nice levels that can be applied to userspace processes.
============     ===========