zpool-features —
ZFS pool feature descriptions
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.
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.
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.
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).
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.
Some features depend on other features being enabled in order to function
properly. Enabling a feature will automatically enable any features it depends
on.
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.
- zpool_checkpoint
-
| GUID |
com.delphix:zpool_checkpoint |
| READ-ONLY COMPATIBLE |
yes |
| DEPENDENCIES |
none |
This feature enables the "zpool checkpoint" subcommand that can
checkpoint the state of the pool at the time it was issued and later
rewind back to it or discard it.
This feature becomes active when the
"zpool checkpoint" command is used to checkpoint the pool. The
feature will only return back to being
enabled when the pool is rewound or the
checkpoint has been discarded.
- device_removal
-
| GUID |
com.delphix:device_removal |
| READ-ONLY COMPATIBLE |
no |
| DEPENDENCIES |
none |
This feature enables the "zpool remove" subcommand to remove
top-level vdevs, evacuating them to reduce the total size of the pool.
This feature becomes active when the
"zpool remove" command is used on a top-level vdev, and will
never return to being enabled.
- obsolete_counts
-
| GUID |
com.delphix:obsolete_counts |
| READ-ONLY COMPATIBLE |
yes |
| DEPENDENCIES |
device_removal |
This feature is an enhancement of device_removal, which will over time
reduce the memory used to track removed devices. When indirect blocks are
freed or remapped, we note that their part of the indirect mapping is
"obsolete", i.e. no longer needed. See also the "zfs
remap" subcommand in
zfs(8).
This feature becomes active when the
"zpool remove" command is used on a top-level vdev, and will
never return to being enabled.
- spacemap_v2
-
| GUID |
com.delphix:spacemap_v2 |
| READ-ONLY COMPATIBLE |
yes |
| DEPENDENCIES |
none |
This feature enables the use of the new space map encoding which consists of
two words (instead of one) whenever it is advantageous. The new encoding
allows space maps to represent large regions of space more efficiently
on-disk while also increasing their maximum addressable offset.
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.
- 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.
zpool(8)
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⟩.
Visit the GSP FreeBSD Man Page Interface.