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ZPOOL-FEATURES(7) 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. 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.
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.
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.
Please note that booting from datasets that have recordsize
greater than 128KB is NOT supported by the FreeBSD boot loader.
sha512
GUID org.illumos:sha512
READ-ONLY COMPATIBLE no
DEPENDENCIES none
The sha512 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 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 dataset
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 a pools utilizing SHA-512/256 is NOT yet
supported.
skein
GUID org.illumos:skein
READ-ONLY COMPATIBLE no
DEPENDENCIES none
The skein 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 dataset
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.
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>.
NetBSD 10.99 May 31, 2016 NetBSD 10.99