Provided by: zfsutils-linux_0.7.9-3ubuntu6_amd64 bug

NAME

       zpool-features - ZFS pool feature descriptions

DESCRIPTION

       ZFS  pool  on-disk  format  versions  are  specified  via "features" which replace the old
       on-disk format numbers (the last supported on-disk format  number  is  28).  To  enable  a
       feature  on  a  pool  use  the  upgrade  subcommand  of  the  zpool(8) command, or set the
       feature@feature_name property to enabled.

       The pool format does not affect file system version compatibility or the ability  to  send
       file systems between pools.

       Since  most  features can be enabled independently of each other the on-disk format of the
       pool is specified by the set of all features marked as active on the pool. If the pool was
       created by another software version this set may include unsupported features.

   Identifying features
       Every  feature  has  a  guid  of  the  form com.example:feature_name. The reverse DNS name
       ensures that the feature's guid is unique across all ZFS implementations. When unsupported
       features  are  encountered  on a pool they will be identified by their guids. Refer to the
       documentation for the ZFS implementation that created the pool for information about those
       features.

       Each  supported feature also has a short name. By convention a feature's short name is the
       portion of its guid which follows the ':' (e.g.  com.example:feature_name would  have  the
       short   name  feature_name),  however  a  feature's  short  name  may  differ  across  ZFS
       implementations if following the convention would result in name conflicts.

   Feature states
       Features can be in one of three states:

       active
                   This feature's on-disk format changes are in effect on the pool.  Support  for
                   this  feature  is  required  to  import  the  pool in read-write mode. If this
                   feature is not read-only compatible, support is also required  to  import  the
                   pool in read-only mode (see "Read-only compatibility").

       enabled
                   An  administrator  has  marked  this  feature  as enabled on the pool, but the
                   feature's on-disk format changes have not been made yet. The pool can still be
                   imported  by  software  that does not support this feature, but changes may be
                   made to the on-disk format at any time which will  move  the  feature  to  the
                   active  state.  Some features may support returning to the enabled state after
                   becoming active. See feature-specific documentation for details.

       disabled
                   This feature's on-disk format changes have not been made and will not be  made
                   unless  an  administrator  moves  the  feature  to the enabled state. Features
                   cannot be disabled once they have been enabled.

       The  state  of  supported  features  is  exposed  through  pool  properties  of  the  form
       feature@short_name.

   Read-only compatibility
       Some  features may make on-disk format changes that do not interfere with other software's
       ability to read from the pool. These features are referred to as  "read-only  compatible".
       If  all  unsupported features on a pool are read-only compatible, the pool can be imported
       in read-only mode by setting the readonly property during import (see zpool(8) for details
       on importing pools).

   Unsupported features
       For  each  unsupported  feature  enabled  on  an  imported  pool  a  pool  property  named
       unsupported@feature_guid will indicate why the import was allowed despite the  unsupported
       feature. Possible values for this property are:

       inactive
                   The feature is in the enabled state and therefore the pool's on-disk format is
                   still compatible with software that does not support this feature.

       readonly
                   The feature is  read-only  compatible  and  the  pool  has  been  imported  in
                   read-only mode.

   Feature dependencies
       Some  features  depend  on  other  features  being  enabled in order to function properly.
       Enabling a feature will automatically enable any features it depends on.

FEATURES

       The following features are supported on this system:

       async_destroy

           GUID                   com.delphix:async_destroy
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           none

           Destroying a file system requires traversing all of its data in order  to  return  its
           used  space  to  the  pool. Without async_destroy the file system is not fully removed
           until all space has been reclaimed. If the  destroy  operation  is  interrupted  by  a
           reboot  or  power  outage  the next attempt to open the pool will need to complete the
           destroy operation synchronously.

           When async_destroy is enabled the file system's data will be reclaimed by a background
           process, allowing the destroy operation to complete without traversing the entire file
           system. The background process is able to resume interrupted destroys after  the  pool
           has  been  opened,  eliminating the need to finish interrupted destroys as part of the
           open operation. The amount of space  remaining  to  be  reclaimed  by  the  background
           process is available through the freeing property.

           This feature is only active while freeing is non-zero.

       empty_bpobj

           GUID                   com.delphix:empty_bpobj
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           none

           This  feature  increases  the  performance  of  creating  and  using a large number of
           snapshots of a single filesystem or volume, and also reduces the disk space required.

           When there are many snapshots, each snapshot uses many Block Pointer Objects (bpobj's)
           to  track blocks associated with that snapshot.  However, in common use cases, most of
           these bpobj's are empty.  This feature allows us to create each bpobj on-demand,  thus
           eliminating the empty bpobjs.

           This  feature  is  active while there are any filesystems, volumes, or snapshots which
           were created after enabling this feature.

       filesystem_limits

           GUID                   com.joyent:filesystem_limits
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           extensible_dataset

           This feature enables filesystem and snapshot limits.  These  limits  can  be  used  to
           control  how many filesystems and/or snapshots can be created at the point in the tree
           on which the limits are set.

           This feature is active once either of the limit properties has been set on a  dataset.
           Once activated the feature is never deactivated.

       lz4_compress

           GUID                   org.illumos:lz4_compress
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           none

           lz4  is a high-performance real-time compression algorithm that features significantly
           faster compression and decompression as well as a higher compression  ratio  than  the
           older  lzjb  compression.   Typically,  lz4 compression is approximately 50% faster on
           compressible data and 200% faster  on  incompressible  data  than  lzjb.  It  is  also
           approximately  80%  faster  on  decompression,  while  giving approximately 10% better
           compression ratio.

           When the lz4_compress feature is set to enabled, the administrator  can  turn  on  lz4
           compression  on  any  dataset  on  the pool using the zfs(8) command. Please note that
           doing so will immediately activate the lz4_compress feature  on  the  underlying  pool
           using  the  zfs(1M)  command. Also, all newly written metadata will be compressed with
           lz4 algorithm. Since this feature is not read-only  compatible,  this  operation  will
           render  the pool unimportable on systems without support for the lz4_compress feature.
           Booting off of lz4-compressed root pools is supported.

           This feature becomes active as soon as it is enabled and will never  return  to  being
           enabled.

       spacemap_histogram

           GUID                   com.delphix:spacemap_histogram
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           none

           This  features  allows  ZFS  to  maintain  more  information  about  how free space is
           organized within the pool. If this feature is enabled, ZFS will set  this  feature  to
           active  when a new space map object is created or an existing space map is upgraded to
           the new format. Once the feature is active, it will remain in  that  state  until  the
           pool is destroyed.

       multi_vdev_crash_dump

           GUID    com.joyent:multi_vdev_crash_dump
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES    none

           This  feature  allows a dump device to be configured with a pool comprised of multiple
           vdevs.  Those vdevs may be arranged in any mirrored or raidz configuration.

           When the multi_vdev_crash_dump feature is set to enabled, the  administrator  can  use
           the  dumpadm(1M)  command  to  configure a dump device on a pool comprised of multiple
           vdevs.

           Under Linux this feature is registered for compatibility  but  not  used.   New  pools
           created under Linux will have the feature enabled but will never transition to active.
           This functionality is not required in  order  to  support  crash  dumps  under  Linux.
           Existing pools where this feature is active can be imported.

       extensible_dataset

           GUID                   com.delphix:extensible_dataset
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           none

           This  feature allows more flexible use of internal ZFS data structures, and exists for
           other features to depend on.

           This feature will be active when the first dependent feature  uses  it,  and  will  be
           returned to the enabled state when all datasets that use this feature are destroyed.

       bookmarks

           GUID                   com.delphix:bookmarks
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           extensible_dataset

           This feature enables use of the zfs bookmark subcommand.

           This  feature  is  active while any bookmarks exist in the pool.  All bookmarks in the
           pool can be listed by running zfs list -t bookmark -r poolname.

       enabled_txg

           GUID                   com.delphix:enabled_txg
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           none

           Once this feature is enabled ZFS records the transaction group  number  in  which  new
           features  are  enabled. This has no user-visible impact, but other features may depend
           on this feature.

           This feature becomes active as soon as it is enabled and will never  return  to  being
           enabled.

       hole_birth

           GUID                   com.delphix:hole_birth
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           enabled_txg

           This  feature  improves  performance of incremental sends ("zfs send -i") and receives
           for objects with many holes. The most common case of hole-filled objects is zvols.

           An incremental send stream from snapshot A to snapshot B  contains  information  about
           every  block  that  changed between A and B. Blocks which did not change between those
           snapshots can be identified and omitted from the stream  using  a  piece  of  metadata
           called  the  'block  birth  time',  but birth times are not recorded for holes (blocks
           filled only with zeroes). Since holes created after A  cannot  be  distinguished  from
           holes  created before A, information about every hole in the entire filesystem or zvol
           is included in the send stream.

           For workloads where holes are rare this is not a problem. However, when  incrementally
           replicating  filesystems  or  zvols with many holes (for example a zvol formatted with
           another filesystem) a lot of time will be  spent  sending  and  receiving  unnecessary
           information about holes that already exist on the receiving side.

           Once  the  hole_birth  feature has been enabled the block birth times of all new holes
           will be recorded. Incremental sends between snapshots created after  this  feature  is
           enabled  will  use  this  new  metadata  to avoid sending information about holes that
           already exist on the receiving side.

           This feature becomes active as soon as it is enabled and will never  return  to  being
           enabled.

       embedded_data

           GUID                   com.delphix:embedded_data
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           none

           This  feature  improves  the  performance and compression ratio of highly-compressible
           blocks.  Blocks whose contents can compress to 112 bytes or smaller can take advantage
           of this feature.

           When this feature is enabled, the contents of highly-compressible blocks are stored in
           the block "pointer" itself (a misnomer in this case, as  it  contains  the  compressed
           data,  rather  than  a  pointer to its location on disk).  Thus the space of the block
           (one sector, typically 512 bytes or 4KB) is saved, and no additional i/o is needed  to
           read and write the data block.

           This  feature  becomes  active as soon as it is enabled and will never return to being
           enabled.

       large_blocks

           GUID                   org.open-zfs:large_block
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           extensible_dataset

           The large_block feature allows the record size on a dataset  to  be  set  larger  than
           128KB.

           This feature becomes active once a recordsize property has been set larger than 128KB,
           and will return to being enabled  once  all  filesystems  that  have  ever  had  their
           recordsize larger than 128KB are destroyed.

       large_dnode

           GUID                   org.zfsonlinux:large_dnode
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           extensible_dataset

           The  large_dnode  feature allows the size of dnodes in a dataset to be set larger than
           512B.

           This feature becomes active once a dataset contains an object with a dnode larger than
           512B,  which  occurs  as a result of setting the dnodesize dataset property to a value
           other than legacy. The feature will return to being enabled once all filesystems  that
           have  ever  contained  a dnode larger than 512B are destroyed. Large dnodes allow more
           data to be stored in the bonus  buffer,  thus  potentially  improving  performance  by
           avoiding the use of spill blocks.

       sha512

           GUID                   org.illumos:sha512

           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           extensible_dataset

           This  feature enables the use of the SHA-512/256 truncated hash algorithm (FIPS 180-4)
           for  checksum  and  dedup.  The  native  64-bit  arithmetic  of  SHA-512  provides  an
           approximate  50%  performance boost over SHA-256 on 64-bit hardware and is thus a good
           minimum-change replacement candidate for systems where hash performance is  important,
           but  these  systems  cannot  for  whatever  reason  utilize the faster skein and edonr
           algorithms.

           When the sha512 feature is set to enabled, the administrator can turn  on  the  sha512
           checksum  on  any dataset using the zfs set checksum=sha512(1M) command.  This feature
           becomes active once a checksum property has been set to sha512,  and  will  return  to
           being enabled once all filesystems that have ever had their checksum set to sha512 are
           destroyed.

           Booting off of pools utilizing SHA-512/256 is supported  (provided  that  the  updated
           GRUB stage2 module is installed).

       skein

           GUID                   org.illumos:skein
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           extensible_dataset

           This feature enables the use of the Skein hash algorithm for checksum and dedup. Skein
           is a high-performance secure hash algorithm that was a  finalist  in  the  NIST  SHA-3
           competition.  It  provides  a very high security margin and high performance on 64-bit
           hardware (80% faster than SHA-256). This implementation also utilizes the  new  salted
           checksumming  functionality in ZFS, which means that the checksum is pre-seeded with a
           secret 256-bit random key (stored on the pool) before being fed the data block  to  be
           checksummed.  Thus  the produced checksums are unique to a given pool, preventing hash
           collision attacks on systems with dedup.

           When the skein feature is set to enabled, the administrator  can  turn  on  the  skein
           checksum  on  any  dataset using the zfs set checksum=skein(1M) command.  This feature
           becomes active once a checksum property has been set to  skein,  and  will  return  to
           being  enabled once all filesystems that have ever had their checksum set to skein are
           destroyed.

           Booting off of pools using skein is NOT supported -- any attempt to enable skein on  a
           root pool will fail with an error.

       edonr

           GUID                   org.illumos:edonr
           READ-ONLY COMPATIBLE   no
           DEPENDENCIES           extensible_dataset

           This  feature enables the use of the Edon-R hash algorithm for checksum, including for
           nopwrite (if compression is also enabled, an  overwrite  of  a  block  whose  checksum
           matches  the  data being written will be ignored).  In an abundance of caution, Edon-R
           can not be used with dedup (without verification).

           Edon-R is a very high-performance hash algorithm that  was  part  of  the  NIST  SHA-3
           competition.  It  provides  extremely  high  hash  performance  (over 350% faster than
           SHA-256), but was not selected because of  its  unsuitability  as  a  general  purpose
           secure  hash  algorithm.   This  implementation  utilizes  the new salted checksumming
           functionality in ZFS, which means that  the  checksum  is  pre-seeded  with  a  secret
           256-bit  random  key  (stored  on  the  pool)  before  being  fed the data block to be
           checksummed. Thus the produced checksums are unique to a given pool.

           When the edonr feature is set to enabled, the administrator  can  turn  on  the  edonr
           checksum  on  any  dataset using the zfs set checksum=edonr(1M) command.  This feature
           becomes active once a checksum property has been set to  edonr,  and  will  return  to
           being  enabled once all filesystems that have ever had their checksum set to edonr are
           destroyed.

           Booting off of pools using edonr is NOT supported -- any attempt to enable edonr on  a
           root pool will fail with an error.

       userobj_accounting

           GUID                   org.zfsonlinux:userobj_accounting
           READ-ONLY COMPATIBLE   yes
           DEPENDENCIES           extensible_dataset

           This feature allows administrators to account the object usage information by user and
           group.

           This feature becomes active as soon as it is enabled and will never  return  to  being
           enabled.  Each  filesystem  will be upgraded automatically when remounted, or when new
           files are created under that filesystem.  The upgrade can also be started manually  on
           filesystems  by  running `zfs set version=current <pool/fs>`. The upgrade process runs
           in the background and may take a while to complete for filesystems containing a  large
           number of files.

SEE ALSO

       zpool(8)

                                           Aug 27, 2013                         ZPOOL-FEATURES(5)