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ZPOOL-FEATURES(7) 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:

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).
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.
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 @feature-name will indicate why the import was allowed despite the unsupported feature. Possible values for this property are:

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.
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 (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# 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:

GUID
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.

GUID
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.

GUID
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 -r poolname.

GUID
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.

GUID
DEPENDENCIES
bookmark, extensible_dataset, bookmark_v2
READ-ONLY COMPATIBLE
no

This feature enables additional bookmark accounting fields, enabling the #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.

GUID
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.

GUID
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.

GUID
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.

GUID
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 =edonr, (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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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 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.

GUID
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.

GUID
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 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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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 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.

GUID
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).

GUID
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).

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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.

GUID
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 , 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)

May 31, 2021 OpenZFS