ZPOOL-FEATURES(7) FreeBSD Miscellaneous Information Manual ZPOOL-FEATURES(7)
NAME
zpool-features - description of ZFS pool features
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 zpool upgrade, or
set the feature@feature-name property to enabled. Please also see the
Compatibility feature sets section for information on how sets of
features may be enabled together.
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
reversed 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 `:' (i.e.
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-import(8) for
details on importing pools).
Unsupported features
For each unsupported feature enabled on an imported pool, a pool property
named unsupported@feature-name 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. Enabling a feature will automatically enable any features it
depends on.
Compatibility feature sets
It is sometimes necessary for a pool to maintain compatibility with a
specific on-disk format, by enabling and disabling particular features.
The compatibility feature facilitates this by allowing feature sets to be
read from text files. When set to off (the default), compatibility
feature sets are disabled (i.e. all features are enabled); when set to
legacy, no features are enabled. When set to a comma-separated list of
filenames (each filename may either be an absolute path, or relative to
/etc/zfs/compatibility.d or /usr/share/zfs/compatibility.d), the lists of
requested features are read from those files, separated by whitespace
and/or commas. Only features present in all files are enabled.
Simple sanity checks are applied to the files: they must be between 1B
and 16kB in size, and must end with a newline character.
The requested features are applied when a pool is created using zpool
create -o compatibility=<?> and controls which features are enabled when
using zpool upgrade. zpool status will not show a warning about disabled
features which are not part of the requested feature set.
The special value legacy prevents any features from being enabled, either
via zpool upgrade or zpool set feature@feature-name=enabled. This
setting also prevents pools from being upgraded to newer on-disk
versions. This is a safety measure to prevent new features from being
accidentally enabled, breaking compatibility.
By convention, compatibility files in /usr/share/zfs/compatibility.d are
provided by the distribution, and include feature sets supported by
important versions of popular distributions, and feature sets commonly
supported at the start of each year. Compatibility files in
/etc/zfs/compatibility.d, if present, will take precedence over files
with the same name in /usr/share/zfs/compatibility.d.
If an unrecognized feature is found in these files, an error message will
be shown. If the unrecognized feature is in a file in
/etc/zfs/compatibility.d, this is treated as an error and processing will
stop. If the unrecognized feature is under
/usr/share/zfs/compatibility.d, this is treated as a warning and
processing will continue. This difference is to allow distributions to
include features which might not be recognized by the currently-installed
binaries.
Compatibility files may include comments: any text from `#' to the end of
the line is ignored.
Example:
example# cat /usr/share/zfs/compatibility.d/grub2
# Features which are supported by GRUB2
async_destroy
bookmarks
embedded_data
empty_bpobj
enabled_txg
extensible_dataset
filesystem_limits
hole_birth
large_blocks
lz4_compress
spacemap_histogram
example# zpool create -o compatibility=grub2 bootpool vdev
See zpool-create(8) and zpool-upgrade(8) for more information on how
these commands are affected by feature sets.
FEATURES
The following features are supported on this system:
allocation_classes
GUID org.zfsonlinux:allocation_classes
READ-ONLY COMPATIBLE yes
This feature enables support for separate allocation classes.
This feature becomes active when a dedicated allocation class
vdev (dedup or special) is created with the zpool create or zpool
add commands. With device removal, it can be returned to the
enabled state if all the dedicated allocation class vdevs are
removed.
async_destroy
GUID com.delphix:async_destroy
READ-ONLY COMPATIBLE yes
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.
bookmarks
GUID com.delphix:bookmarks
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE yes
This feature enables use of the zfs bookmark command.
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.
bookmark_v2
GUID com.datto:bookmark_v2
DEPENDENCIES bookmark, extensible_dataset
READ-ONLY COMPATIBLE no
This feature enables the creation and management of larger
bookmarks which are needed for other features in ZFS.
This feature becomes active when a v2 bookmark is created and
will be returned to the enabled state when all v2 bookmarks are
destroyed.
bookmark_written
GUID com.delphix:bookmark_written
DEPENDENCIES bookmark, extensible_dataset, bookmark_v2
READ-ONLY COMPATIBLE no
This feature enables additional bookmark accounting fields,
enabling the written#bookmark property (space written since a
bookmark) and estimates of send stream sizes for incrementals
from bookmarks.
This feature becomes active when a bookmark is created and will
be returned to the enabled state when all bookmarks with these
fields are destroyed.
device_rebuild
GUID org.openzfs:device_rebuild
READ-ONLY COMPATIBLE yes
This feature enables the ability for the zpool attach and zpool
replace commands to perform sequential reconstruction (instead of
healing reconstruction) when resilvering.
Sequential reconstruction resilvers a device in LBA order without
immediately verifying the checksums. Once complete, a scrub is
started, which then verifies the checksums. This approach allows
full redundancy to be restored to the pool in the minimum amount
of time. This two-phase approach will take longer than a healing
resilver when the time to verify the checksums is included.
However, unless there is additional pool damage, no checksum
errors should be reported by the scrub. This feature is
incompatible with raidz configurations. This feature becomes
active while a sequential resilver is in progress, and returns to
enabled when the resilver completes.
device_removal
GUID com.delphix:device_removal
READ-ONLY COMPATIBLE no
This feature enables the zpool remove command 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.
draid
GUID org.openzfs:draid
READ-ONLY COMPATIBLE no
This feature enables use of the draid vdev type. dRAID is a
variant of raidz which provides integrated distributed hot spares
that allow faster resilvering while retaining the benefits of
raidz. Data, parity, and spare space are organized in redundancy
groups and distributed evenly over all of the devices.
This feature becomes active when creating a pool which uses the
draid vdev type, or when adding a new draid vdev to an existing
pool.
edonr
GUID org.illumos:edonr
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
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
requires verification when used with dedup: zfs set
dedup=edonr,verify (see zfs-set(8)).
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, preventing
hash collision attacks on systems with dedup.
When the edonr feature is set to enabled, the administrator can
turn on the edonr checksum on any dataset using zfs set
checksum=edonr dset (see zfs-set(8)). 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.
FreeBSD does not support the edonr feature.
embedded_data
GUID com.delphix:embedded_data
READ-ONLY COMPATIBLE no
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 512B 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.
empty_bpobj
GUID com.delphix:empty_bpobj
READ-ONLY COMPATIBLE yes
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 (bpobjs) to track blocks associated with that
snapshot. However, in common use cases, most of these bpobjs 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.
enabled_txg
GUID com.delphix:enabled_txg
READ-ONLY COMPATIBLE yes
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.
encryption
GUID com.datto:encryption
DEPENDENCIES bookmark_v2, extensible_dataset
READ-ONLY COMPATIBLE no
This feature enables the creation and management of natively
encrypted datasets.
This feature becomes active when an encrypted dataset is created
and will be returned to the enabled state when all datasets that
use this feature are destroyed.
extensible_dataset
GUID com.delphix:extensible_dataset
READ-ONLY COMPATIBLE no
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.
filesystem_limits
GUID com.joyent:filesystem_limits
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE yes
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.
hole_birth
GUID com.delphix:hole_birth
DEPENDENCIES enabled_txg
READ-ONLY COMPATIBLE no
This feature has/had bugs, the result of which is that, if you do
a zfs send -i (or -R, since it uses -i) from an affected dataset,
the receiving party will not see any checksum or other errors,
but the resulting destination snapshot will not match the source.
Its use by zfs send -i has been disabled by default (see
send_holes_without_birth_time in zfs(4)).
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.
large_blocks
GUID org.open-zfs:large_blocks
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
This feature allows the record size on a dataset to be set larger
than 128kB.
This feature becomes active once a dataset contains a file with a
block size 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
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
This 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.
livelist
GUID com.delphix:livelist
READ-ONLY COMPATIBLE yes
This feature allows clones to be deleted faster than the
traditional method when a large number of random/sparse writes
have been made to the clone. All blocks allocated and freed
after a clone is created are tracked by the the clone's livelist
which is referenced during the deletion of the clone. The
feature is activated when a clone is created and remains active
until all clones have been destroyed.
log_spacemap
GUID com.delphix:log_spacemap
DEPENDENCIES com.delphix:spacemap_v2
READ-ONLY COMPATIBLE yes
This feature improves performance for heavily-fragmented pools,
especially when workloads are heavy in random-writes. It does so
by logging all the metaslab changes on a single spacemap every
TXG instead of scattering multiple writes to all the metaslab
spacemaps.
This feature becomes active as soon as it is enabled and will
never return to being enabled.
lz4_compress
GUID org.illumos:lz4_compress
READ-ONLY COMPATIBLE no
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 a 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-set(8) command. All newly written metadata
will be compressed with the lz4 algorithm.
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
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 dumpadm(1M) to configure a dump device on a
pool comprised of multiple vdevs.
Under FreeBSD and Linux this feature is unused, but registered
for compatibility. New pools created on these systems will have
the feature enabled but will never transition to active, as this
functionality is not required for crash dump support. Existing
pools where this feature is active can be imported.
obsolete_counts
GUID com.delphix:obsolete_counts
DEPENDENCIES device_removal
READ-ONLY COMPATIBLE yes
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" - no longer needed.
This feature becomes active when the zpool remove command is used
on a top-level vdev, and will never return to being enabled.
project_quota
GUID org.zfsonlinux:project_quota
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE yes
This feature allows administrators to account the spaces and
objects usage information against the project identifier (ID).
The project ID is an object-based attribute. When upgrading an
existing filesystem, objects without a project ID will be
assigned a zero project ID. When this feature is enabled, newly
created objects inherit their parent directories' project ID if
the parent's inherit flag is set (via chattr [+-]P or zfs project
-s|-C). Otherwise, the new object's project ID will be zero. An
object's project ID can be changed at any time by the owner (or
privileged user) via chattr -p prjid or zfs project -p prjid.
This feature will become active as soon as it is enabled and will
never return to being disabled. Each filesystem will be upgraded
automatically when remounted, or when a new file is created under
that filesystem. The upgrade can also be triggered on filesystems
via zfs set version=current fs. The upgrade process runs in the
background and may take a while to complete for filesystems
containing large amounts of files.
redaction_bookmarks
GUID com.delphix:redaction_bookmarks
DEPENDENCIES bookmarks, extensible_dataset
READ-ONLY COMPATIBLE no
This feature enables the use of redacted zfs sends, which create
redaction bookmarks storing the list of blocks redacted by the
send that created them. For more information about redacted
sends, see zfs-send(8).
redacted_datasets
GUID com.delphix:redacted_datasets
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
This feature enables the receiving of redacted zfs sendstreams.
which create redacted datasets when received. These datasets are
missing some of their blocks, and so cannot be safely mounted,
and their contents cannot be safely read. For more information
about redacted receives, see zfs-send(8).
resilver_defer
GUID com.datto:resilver_defer
READ-ONLY COMPATIBLE yes
This feature allows ZFS to postpone new resilvers if an existing
one is already in progress. Without this feature, any new
resilvers will cause the currently running one to be immediately
restarted from the beginning.
This feature becomes active once a resilver has been deferred,
and returns to being enabled when the deferred resilver begins.
sha512
GUID org.illumos:sha512
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
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 zfs set
checksum=sha512 dset (see zfs-set(8)). 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
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
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 zfs set
checksum=skein dset (see zfs-set(8)). 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.
spacemap_histogram
GUID com.delphix:spacemap_histogram
READ-ONLY COMPATIBLE yes
This features allows ZFS to maintain more information about how
free space is organized within the pool. If this feature is
enabled, it will be activated when a new space map object is
created, or an existing space map is upgraded to the new format,
and never returns back to being enabled.
spacemap_v2
GUID com.delphix:spacemap_v2
READ-ONLY COMPATIBLE yes
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 once it is enabled, and never returns
back to being enabled.
userobj_accounting
GUID org.zfsonlinux:userobj_accounting
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE yes
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 a new file is created under
that filesystem. The upgrade can also be triggered on filesystems
via zfs set version=current fs. The upgrade process runs in the
background and may take a while to complete for filesystems
containing large amounts of files.
zpool_checkpoint
GUID com.delphix:zpool_checkpoint
READ-ONLY COMPATIBLE yes
This feature enables the zpool checkpoint command 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.
zstd_compress
GUID org.freebsd:zstd_compress
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no
zstd is a high-performance compression algorithm that features a
combination of high compression ratios and high speed. Compared
to gzip, zstd offers slightly better compression at much higher
speeds. Compared to lz4, zstd offers much better compression
while being only modestly slower. Typically, zstd compression
speed ranges from 250 to 500 MB/s per thread and decompression
speed is over 1 GB/s per thread.
When the zstd feature is set to enabled, the administrator can
turn on zstd compression of any dataset using zfs set
compress=zstd dset (see zfs-set(8)). This feature becomes active
once a compress property has been set to zstd, and will return to
being enabled once all filesystems that have ever had their
compress property set to zstd are destroyed.
SEE ALSO
zpool(8)
FreeBSD 13.1-RELEASE-p6 May 31, 2021 FreeBSD 13.1-RELEASE-p6
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