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ZPOOL-FEATURES(7)  FreeBSD Miscellaneous Information Manual  ZPOOL-FEATURES(7)

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 (even before any data is written), and the feature will
               not be deactivated.  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.

     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.

     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.

     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.

SEE ALSO
     zpool(8)

AUTHORS
     This manual page is a mdoc(7) reimplementation of the illumos manual page
     zpool-features(5), modified and customized for FreeBSD and licensed under
     the Common Development and Distribution License (CDDL).

     The mdoc(7) implementation of this manual page was initially written by
     Martin Matuska <mm@FreeBSD.org>.

FreeBSD 11.0-PRERELEASE         April 23, 2014         FreeBSD 11.0-PRERELEASE

NAME | DESCRIPTION | FEATURES | SEE ALSO | AUTHORS

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