Ubuntu Manpage: zpool-features — description of ZFS pool features

Provided by: zfsutils-linux_2.1.5-1ubuntu6_amd64

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.

NAME

DESCRIPTION

FEATURES

SEE ALSO