Provided by: zfsutils-linux_0.6.5.6-0ubuntu30_amd64 
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.
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 compresseed
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.
SEE ALSO
zpool(8)
Aug 27, 2013 ZPOOL-FEATURES(5)