zpool-features —
description of ZFS pool features
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.
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.
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-import(8) for details on importing pools).
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.
Some features depend on other features being enabled in order to function.
Enabling a feature will automatically enable any features it depends on.
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.
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.
zpool(8)