1.. SPDX-License-Identifier: GPL-2.0
2
3======================
4The SGI XFS Filesystem
5======================
6
7XFS is a high performance journaling filesystem which originated
8on the SGI IRIX platform.  It is completely multi-threaded, can
9support large files and large filesystems, extended attributes,
10variable block sizes, is extent based, and makes extensive use of
11Btrees (directories, extents, free space) to aid both performance
12and scalability.
13
14Refer to the documentation at https://xfs.wiki.kernel.org/
15for further details.  This implementation is on-disk compatible
16with the IRIX version of XFS.
17
18
19Mount Options
20=============
21
22When mounting an XFS filesystem, the following options are accepted.
23
24  allocsize=size
25	Sets the buffered I/O end-of-file preallocation size when
26	doing delayed allocation writeout (default size is 64KiB).
27	Valid values for this option are page size (typically 4KiB)
28	through to 1GiB, inclusive, in power-of-2 increments.
29
30	The default behaviour is for dynamic end-of-file
31	preallocation size, which uses a set of heuristics to
32	optimise the preallocation size based on the current
33	allocation patterns within the file and the access patterns
34	to the file. Specifying a fixed ``allocsize`` value turns off
35	the dynamic behaviour.
36
37  attr2 or noattr2
38	The options enable/disable an "opportunistic" improvement to
39	be made in the way inline extended attributes are stored
40	on-disk.  When the new form is used for the first time when
41	``attr2`` is selected (either when setting or removing extended
42	attributes) the on-disk superblock feature bit field will be
43	updated to reflect this format being in use.
44
45	The default behaviour is determined by the on-disk feature
46	bit indicating that ``attr2`` behaviour is active. If either
47	mount option is set, then that becomes the new default used
48	by the filesystem.
49
50	CRC enabled filesystems always use the ``attr2`` format, and so
51	will reject the ``noattr2`` mount option if it is set.
52
53  discard or nodiscard (default)
54	Enable/disable the issuing of commands to let the block
55	device reclaim space freed by the filesystem.  This is
56	useful for SSD devices, thinly provisioned LUNs and virtual
57	machine images, but may have a performance impact.
58
59	Note: It is currently recommended that you use the ``fstrim``
60	application to ``discard`` unused blocks rather than the ``discard``
61	mount option because the performance impact of this option
62	is quite severe.
63
64  grpid/bsdgroups or nogrpid/sysvgroups (default)
65	These options define what group ID a newly created file
66	gets.  When ``grpid`` is set, it takes the group ID of the
67	directory in which it is created; otherwise it takes the
68	``fsgid`` of the current process, unless the directory has the
69	``setgid`` bit set, in which case it takes the ``gid`` from the
70	parent directory, and also gets the ``setgid`` bit set if it is
71	a directory itself.
72
73  filestreams
74	Make the data allocator use the filestreams allocation mode
75	across the entire filesystem rather than just on directories
76	configured to use it.
77
78  ikeep or noikeep (default)
79	When ``ikeep`` is specified, XFS does not delete empty inode
80	clusters and keeps them around on disk.  When ``noikeep`` is
81	specified, empty inode clusters are returned to the free
82	space pool.
83
84  inode32 or inode64 (default)
85	When ``inode32`` is specified, it indicates that XFS limits
86	inode creation to locations which will not result in inode
87	numbers with more than 32 bits of significance.
88
89	When ``inode64`` is specified, it indicates that XFS is allowed
90	to create inodes at any location in the filesystem,
91	including those which will result in inode numbers occupying
92	more than 32 bits of significance.
93
94	``inode32`` is provided for backwards compatibility with older
95	systems and applications, since 64 bits inode numbers might
96	cause problems for some applications that cannot handle
97	large inode numbers.  If applications are in use which do
98	not handle inode numbers bigger than 32 bits, the ``inode32``
99	option should be specified.
100
101  largeio or nolargeio (default)
102	If ``nolargeio`` is specified, the optimal I/O reported in
103	``st_blksize`` by **stat(2)** will be as small as possible to allow
104	user applications to avoid inefficient read/modify/write
105	I/O.  This is typically the page size of the machine, as
106	this is the granularity of the page cache.
107
108	If ``largeio`` is specified, a filesystem that was created with a
109	``swidth`` specified will return the ``swidth`` value (in bytes)
110	in ``st_blksize``. If the filesystem does not have a ``swidth``
111	specified but does specify an ``allocsize`` then ``allocsize``
112	(in bytes) will be returned instead. Otherwise the behaviour
113	is the same as if ``nolargeio`` was specified.
114
115  logbufs=value
116	Set the number of in-memory log buffers.  Valid numbers
117	range from 2-8 inclusive.
118
119	The default value is 8 buffers.
120
121	If the memory cost of 8 log buffers is too high on small
122	systems, then it may be reduced at some cost to performance
123	on metadata intensive workloads. The ``logbsize`` option below
124	controls the size of each buffer and so is also relevant to
125	this case.
126
127  logbsize=value
128	Set the size of each in-memory log buffer.  The size may be
129	specified in bytes, or in kilobytes with a "k" suffix.
130	Valid sizes for version 1 and version 2 logs are 16384 (16k)
131	and 32768 (32k).  Valid sizes for version 2 logs also
132	include 65536 (64k), 131072 (128k) and 262144 (256k). The
133	logbsize must be an integer multiple of the log
134	stripe unit configured at **mkfs(8)** time.
135
136	The default value for version 1 logs is 32768, while the
137	default value for version 2 logs is MAX(32768, log_sunit).
138
139  logdev=device and rtdev=device
140	Use an external log (metadata journal) and/or real-time device.
141	An XFS filesystem has up to three parts: a data section, a log
142	section, and a real-time section.  The real-time section is
143	optional, and the log section can be separate from the data
144	section or contained within it.
145
146  noalign
147	Data allocations will not be aligned at stripe unit
148	boundaries. This is only relevant to filesystems created
149	with non-zero data alignment parameters (``sunit``, ``swidth``) by
150	**mkfs(8)**.
151
152  norecovery
153	The filesystem will be mounted without running log recovery.
154	If the filesystem was not cleanly unmounted, it is likely to
155	be inconsistent when mounted in ``norecovery`` mode.
156	Some files or directories may not be accessible because of this.
157	Filesystems mounted ``norecovery`` must be mounted read-only or
158	the mount will fail.
159
160  nouuid
161	Don't check for double mounted file systems using the file
162	system ``uuid``.  This is useful to mount LVM snapshot volumes,
163	and often used in combination with ``norecovery`` for mounting
164	read-only snapshots.
165
166  noquota
167	Forcibly turns off all quota accounting and enforcement
168	within the filesystem.
169
170  uquota/usrquota/uqnoenforce/quota
171	User disk quota accounting enabled, and limits (optionally)
172	enforced.  Refer to **xfs_quota(8)** for further details.
173
174  gquota/grpquota/gqnoenforce
175	Group disk quota accounting enabled and limits (optionally)
176	enforced.  Refer to **xfs_quota(8)** for further details.
177
178  pquota/prjquota/pqnoenforce
179	Project disk quota accounting enabled and limits (optionally)
180	enforced.  Refer to **xfs_quota(8)** for further details.
181
182  sunit=value and swidth=value
183	Used to specify the stripe unit and width for a RAID device
184	or a stripe volume.  "value" must be specified in 512-byte
185	block units. These options are only relevant to filesystems
186	that were created with non-zero data alignment parameters.
187
188	The ``sunit`` and ``swidth`` parameters specified must be compatible
189	with the existing filesystem alignment characteristics.  In
190	general, that means the only valid changes to ``sunit`` are
191	increasing it by a power-of-2 multiple. Valid ``swidth`` values
192	are any integer multiple of a valid ``sunit`` value.
193
194	Typically the only time these mount options are necessary if
195	after an underlying RAID device has had it's geometry
196	modified, such as adding a new disk to a RAID5 lun and
197	reshaping it.
198
199  swalloc
200	Data allocations will be rounded up to stripe width boundaries
201	when the current end of file is being extended and the file
202	size is larger than the stripe width size.
203
204  wsync
205	When specified, all filesystem namespace operations are
206	executed synchronously. This ensures that when the namespace
207	operation (create, unlink, etc) completes, the change to the
208	namespace is on stable storage. This is useful in HA setups
209	where failover must not result in clients seeing
210	inconsistent namespace presentation during or after a
211	failover event.
212
213Deprecation of V4 Format
214========================
215
216The V4 filesystem format lacks certain features that are supported by
217the V5 format, such as metadata checksumming, strengthened metadata
218verification, and the ability to store timestamps past the year 2038.
219Because of this, the V4 format is deprecated.  All users should upgrade
220by backing up their files, reformatting, and restoring from the backup.
221
222Administrators and users can detect a V4 filesystem by running xfs_info
223against a filesystem mountpoint and checking for a string containing
224"crc=".  If no such string is found, please upgrade xfsprogs to the
225latest version and try again.
226
227The deprecation will take place in two parts.  Support for mounting V4
228filesystems can now be disabled at kernel build time via Kconfig option.
229The option will default to yes until September 2025, at which time it
230will be changed to default to no.  In September 2030, support will be
231removed from the codebase entirely.
232
233Note: Distributors may choose to withdraw V4 format support earlier than
234the dates listed above.
235
236Deprecated Mount Options
237========================
238
239============================    ================
240  Name				Removal Schedule
241============================    ================
242Mounting with V4 filesystem     September 2030
243Mounting ascii-ci filesystem    September 2030
244ikeep/noikeep			September 2025
245attr2/noattr2			September 2025
246============================    ================
247
248
249Removed Mount Options
250=====================
251
252===========================     =======
253  Name				Removed
254===========================	=======
255  delaylog/nodelaylog		v4.0
256  ihashsize			v4.0
257  irixsgid			v4.0
258  osyncisdsync/osyncisosync	v4.0
259  barrier			v4.19
260  nobarrier			v4.19
261===========================     =======
262
263sysctls
264=======
265
266The following sysctls are available for the XFS filesystem:
267
268  fs.xfs.stats_clear		(Min: 0  Default: 0  Max: 1)
269	Setting this to "1" clears accumulated XFS statistics
270	in /proc/fs/xfs/stat.  It then immediately resets to "0".
271
272  fs.xfs.xfssyncd_centisecs	(Min: 100  Default: 3000  Max: 720000)
273	The interval at which the filesystem flushes metadata
274	out to disk and runs internal cache cleanup routines.
275
276  fs.xfs.filestream_centisecs	(Min: 1  Default: 3000  Max: 360000)
277	The interval at which the filesystem ages filestreams cache
278	references and returns timed-out AGs back to the free stream
279	pool.
280
281  fs.xfs.speculative_prealloc_lifetime
282	(Units: seconds   Min: 1  Default: 300  Max: 86400)
283	The interval at which the background scanning for inodes
284	with unused speculative preallocation runs. The scan
285	removes unused preallocation from clean inodes and releases
286	the unused space back to the free pool.
287
288  fs.xfs.speculative_cow_prealloc_lifetime
289	This is an alias for speculative_prealloc_lifetime.
290
291  fs.xfs.error_level		(Min: 0  Default: 3  Max: 11)
292	A volume knob for error reporting when internal errors occur.
293	This will generate detailed messages & backtraces for filesystem
294	shutdowns, for example.  Current threshold values are:
295
296		XFS_ERRLEVEL_OFF:       0
297		XFS_ERRLEVEL_LOW:       1
298		XFS_ERRLEVEL_HIGH:      5
299
300  fs.xfs.panic_mask		(Min: 0  Default: 0  Max: 511)
301	Causes certain error conditions to call BUG(). Value is a bitmask;
302	OR together the tags which represent errors which should cause panics:
303
304		XFS_NO_PTAG                     0
305		XFS_PTAG_IFLUSH                 0x00000001
306		XFS_PTAG_LOGRES                 0x00000002
307		XFS_PTAG_AILDELETE              0x00000004
308		XFS_PTAG_ERROR_REPORT           0x00000008
309		XFS_PTAG_SHUTDOWN_CORRUPT       0x00000010
310		XFS_PTAG_SHUTDOWN_IOERROR       0x00000020
311		XFS_PTAG_SHUTDOWN_LOGERROR      0x00000040
312		XFS_PTAG_FSBLOCK_ZERO           0x00000080
313		XFS_PTAG_VERIFIER_ERROR         0x00000100
314
315	This option is intended for debugging only.
316
317  fs.xfs.irix_symlink_mode	(Min: 0  Default: 0  Max: 1)
318	Controls whether symlinks are created with mode 0777 (default)
319	or whether their mode is affected by the umask (irix mode).
320
321  fs.xfs.irix_sgid_inherit	(Min: 0  Default: 0  Max: 1)
322	Controls files created in SGID directories.
323	If the group ID of the new file does not match the effective group
324	ID or one of the supplementary group IDs of the parent dir, the
325	ISGID bit is cleared if the irix_sgid_inherit compatibility sysctl
326	is set.
327
328  fs.xfs.inherit_sync		(Min: 0  Default: 1  Max: 1)
329	Setting this to "1" will cause the "sync" flag set
330	by the **xfs_io(8)** chattr command on a directory to be
331	inherited by files in that directory.
332
333  fs.xfs.inherit_nodump		(Min: 0  Default: 1  Max: 1)
334	Setting this to "1" will cause the "nodump" flag set
335	by the **xfs_io(8)** chattr command on a directory to be
336	inherited by files in that directory.
337
338  fs.xfs.inherit_noatime	(Min: 0  Default: 1  Max: 1)
339	Setting this to "1" will cause the "noatime" flag set
340	by the **xfs_io(8)** chattr command on a directory to be
341	inherited by files in that directory.
342
343  fs.xfs.inherit_nosymlinks	(Min: 0  Default: 1  Max: 1)
344	Setting this to "1" will cause the "nosymlinks" flag set
345	by the **xfs_io(8)** chattr command on a directory to be
346	inherited by files in that directory.
347
348  fs.xfs.inherit_nodefrag	(Min: 0  Default: 1  Max: 1)
349	Setting this to "1" will cause the "nodefrag" flag set
350	by the **xfs_io(8)** chattr command on a directory to be
351	inherited by files in that directory.
352
353  fs.xfs.rotorstep		(Min: 1  Default: 1  Max: 256)
354	In "inode32" allocation mode, this option determines how many
355	files the allocator attempts to allocate in the same allocation
356	group before moving to the next allocation group.  The intent
357	is to control the rate at which the allocator moves between
358	allocation groups when allocating extents for new files.
359
360Deprecated Sysctls
361==================
362
363===========================================     ================
364  Name                                          Removal Schedule
365===========================================     ================
366fs.xfs.irix_sgid_inherit                        September 2025
367fs.xfs.irix_symlink_mode                        September 2025
368fs.xfs.speculative_cow_prealloc_lifetime        September 2025
369===========================================     ================
370
371
372Removed Sysctls
373===============
374
375=============================	=======
376  Name				Removed
377=============================	=======
378  fs.xfs.xfsbufd_centisec	v4.0
379  fs.xfs.age_buffer_centisecs	v4.0
380=============================	=======
381
382Error handling
383==============
384
385XFS can act differently according to the type of error found during its
386operation. The implementation introduces the following concepts to the error
387handler:
388
389 -failure speed:
390	Defines how fast XFS should propagate an error upwards when a specific
391	error is found during the filesystem operation. It can propagate
392	immediately, after a defined number of retries, after a set time period,
393	or simply retry forever.
394
395 -error classes:
396	Specifies the subsystem the error configuration will apply to, such as
397	metadata IO or memory allocation. Different subsystems will have
398	different error handlers for which behaviour can be configured.
399
400 -error handlers:
401	Defines the behavior for a specific error.
402
403The filesystem behavior during an error can be set via ``sysfs`` files. Each
404error handler works independently - the first condition met by an error handler
405for a specific class will cause the error to be propagated rather than reset and
406retried.
407
408The action taken by the filesystem when the error is propagated is context
409dependent - it may cause a shut down in the case of an unrecoverable error,
410it may be reported back to userspace, or it may even be ignored because
411there's nothing useful we can with the error or anyone we can report it to (e.g.
412during unmount).
413
414The configuration files are organized into the following hierarchy for each
415mounted filesystem:
416
417  /sys/fs/xfs/<dev>/error/<class>/<error>/
418
419Where:
420  <dev>
421	The short device name of the mounted filesystem. This is the same device
422	name that shows up in XFS kernel error messages as "XFS(<dev>): ..."
423
424  <class>
425	The subsystem the error configuration belongs to. As of 4.9, the defined
426	classes are:
427
428		- "metadata": applies metadata buffer write IO
429
430  <error>
431	The individual error handler configurations.
432
433
434Each filesystem has "global" error configuration options defined in their top
435level directory:
436
437  /sys/fs/xfs/<dev>/error/
438
439  fail_at_unmount		(Min:  0  Default:  1  Max: 1)
440	Defines the filesystem error behavior at unmount time.
441
442	If set to a value of 1, XFS will override all other error configurations
443	during unmount and replace them with "immediate fail" characteristics.
444	i.e. no retries, no retry timeout. This will always allow unmount to
445	succeed when there are persistent errors present.
446
447	If set to 0, the configured retry behaviour will continue until all
448	retries and/or timeouts have been exhausted. This will delay unmount
449	completion when there are persistent errors, and it may prevent the
450	filesystem from ever unmounting fully in the case of "retry forever"
451	handler configurations.
452
453	Note: there is no guarantee that fail_at_unmount can be set while an
454	unmount is in progress. It is possible that the ``sysfs`` entries are
455	removed by the unmounting filesystem before a "retry forever" error
456	handler configuration causes unmount to hang, and hence the filesystem
457	must be configured appropriately before unmount begins to prevent
458	unmount hangs.
459
460Each filesystem has specific error class handlers that define the error
461propagation behaviour for specific errors. There is also a "default" error
462handler defined, which defines the behaviour for all errors that don't have
463specific handlers defined. Where multiple retry constraints are configured for
464a single error, the first retry configuration that expires will cause the error
465to be propagated. The handler configurations are found in the directory:
466
467  /sys/fs/xfs/<dev>/error/<class>/<error>/
468
469  max_retries			(Min: -1  Default: Varies  Max: INTMAX)
470	Defines the allowed number of retries of a specific error before
471	the filesystem will propagate the error. The retry count for a given
472	error context (e.g. a specific metadata buffer) is reset every time
473	there is a successful completion of the operation.
474
475	Setting the value to "-1" will cause XFS to retry forever for this
476	specific error.
477
478	Setting the value to "0" will cause XFS to fail immediately when the
479	specific error is reported.
480
481	Setting the value to "N" (where 0 < N < Max) will make XFS retry the
482	operation "N" times before propagating the error.
483
484  retry_timeout_seconds		(Min:  -1  Default:  Varies  Max: 1 day)
485	Define the amount of time (in seconds) that the filesystem is
486	allowed to retry its operations when the specific error is
487	found.
488
489	Setting the value to "-1" will allow XFS to retry forever for this
490	specific error.
491
492	Setting the value to "0" will cause XFS to fail immediately when the
493	specific error is reported.
494
495	Setting the value to "N" (where 0 < N < Max) will allow XFS to retry the
496	operation for up to "N" seconds before propagating the error.
497
498**Note:** The default behaviour for a specific error handler is dependent on both
499the class and error context. For example, the default values for
500"metadata/ENODEV" are "0" rather than "-1" so that this error handler defaults
501to "fail immediately" behaviour. This is done because ENODEV is a fatal,
502unrecoverable error no matter how many times the metadata IO is retried.
503
504Workqueue Concurrency
505=====================
506
507XFS uses kernel workqueues to parallelize metadata update processes.  This
508enables it to take advantage of storage hardware that can service many IO
509operations simultaneously.  This interface exposes internal implementation
510details of XFS, and as such is explicitly not part of any userspace API/ABI
511guarantee the kernel may give userspace.  These are undocumented features of
512the generic workqueue implementation XFS uses for concurrency, and they are
513provided here purely for diagnostic and tuning purposes and may change at any
514time in the future.
515
516The control knobs for a filesystem's workqueues are organized by task at hand
517and the short name of the data device.  They all can be found in:
518
519  /sys/bus/workqueue/devices/${task}!${device}
520
521================  ===========
522  Task            Description
523================  ===========
524  xfs_iwalk-$pid  Inode scans of the entire filesystem. Currently limited to
525                  mount time quotacheck.
526  xfs-gc          Background garbage collection of disk space that have been
527                  speculatively allocated beyond EOF or for staging copy on
528                  write operations.
529================  ===========
530
531For example, the knobs for the quotacheck workqueue for /dev/nvme0n1 would be
532found in /sys/bus/workqueue/devices/xfs_iwalk-1111!nvme0n1/.
533
534The interesting knobs for XFS workqueues are as follows:
535
536============     ===========
537  Knob           Description
538============     ===========
539  max_active     Maximum number of background threads that can be started to
540                 run the work.
541  cpumask        CPUs upon which the threads are allowed to run.
542  nice           Relative priority of scheduling the threads.  These are the
543                 same nice levels that can be applied to userspace processes.
544============     ===========
545