Provided by: xfsprogs_4.3.0+nmu1ubuntu1_i386 bug


       xfs  - layout, mount options, and supported file attributes for the XFS


       An XFS filesystem can reside on  a  regular  disk  partition  or  on  a
       logical  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,
       directories, 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 parallelism 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  allocation  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
       written  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
       creation,  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
       sector 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
       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  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.


       Some  functionality  specific  to  the  XFS filesystem is accessible to
       applications    through    the    xfsctl(3)    and    by-handle    (see
       open_by_handle(3)) interfaces.


       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.

              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

              CRC enabled filesystems always use the attr2 format, and so will
              reject the noattr2 mount option if it is set.

              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

              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 this case.

              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 mkfs.

              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

       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.

              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 reshaping it.

              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.


       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.


       chattr(1), xfsctl(3), mount(8), mkfs.xfs(8), xfs_info(8), xfs_admin(8),
       xfsdump(8), xfsrestore(8).