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 http://oss.sgi.com/projects/xfs/
  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.
  For boolean mount options, the names with the (*) suffix is the
  default behaviour.
  
    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
    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 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.
  
    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.
  
    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.
  
    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.
  
  
    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 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 time.
  
  	The default value for 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/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.
  
  
  Deprecated Mount Options
  ========================
  
    delaylog/nodelaylog
  	Delayed logging is the only logging method that XFS supports
  	now, so these mount options are now ignored.
  
  	Due for removal in 3.12.
  
    ihashsize=value
  	In memory inode hashes have been removed, so this option has
  	no function as of August 2007. Option is deprecated.
  
  	Due for removal in 3.12.
  
    irixsgid
  	This behaviour is now controlled by a sysctl, so the mount
  	option is ignored.
  
  	Due for removal in 3.12.
  
    osyncisdsync
    osyncisosync
  	O_SYNC and O_DSYNC are fully supported, so there is no need
  	for these options any more.
  
  	Due for removal in 3.12.
  
  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.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: 127)
  	Causes certain error conditions to call BUG(). Value is a bitmask;
  	AND 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
  
  	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
  ==================
  
    fs.xfs.xfsbufd_centisecs	(Min: 50  Default: 100	Max: 3000)
  	Dirty metadata is now tracked by the log subsystem and
  	flushing is driven by log space and idling demands. The
  	xfsbufd no longer exists, so this syctl does nothing.
  
  	Due for removal in 3.14.
  
    fs.xfs.age_buffer_centisecs	(Min: 100  Default: 1500  Max: 720000)
  	Dirty metadata is now tracked by the log subsystem and
  	flushing is driven by log space and idling demands. The
  	xfsbufd no longer exists, so this syctl does nothing.
  
  	Due for removal in 3.14.