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 upgrade subcommand of the zpool(8) command, or set the feature@feature_name property to enabled.

The pool format does not affect file system version compatibility or the ability to send file systems between pools.

Since most features can be enabled independently of each other the on-disk format of the pool is specified by the set of all features marked as active on the pool. If the pool was created by another software version this set may include unsupported features.

Identifying features

Every feature has a guid of the form com.example:feature_name. The reverse DNS name ensures that the feature's guid is unique across all ZFS implementations. When unsupported features are encountered on a pool they will be identified by their guids. Refer to the documentation for the ZFS implementation that created the pool for information about those features.

Each supported feature also has a short name. By convention a feature's short name is the portion of its guid which follows the ':' (e.g. com.example:feature_name would have the short name feature_name), however a feature's short name may differ across ZFS implementations if following the convention would result in name conflicts.

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(8) for details on importing pools).

Unsupported features

For each unsupported feature enabled on an imported pool a pool property named unsupported@feature_guid will indicate why the import was allowed despite the unsupported feature. Possible values for this property are:

inactive

The feature is in the enabled state and therefore the pool's on-disk format is still compatible with software that does not support this feature.

readonly

The feature is read-only compatible and the pool has been imported in read-only mode.

Feature dependencies

Some features depend on other features being enabled in order to function properly. Enabling a feature will automatically enable any features it depends on.

FEATURES

The following features are supported on this system:

async_destroy

GUID	com.delphix:async_destroy
READ-ONLY COMPATIBLE	yes
DEPENDENCIES	none

Destroying a file system requires traversing all of its data in order to return its used space to the pool. Without async_destroy the file system is not fully removed until all space has been reclaimed. If the destroy operation is interrupted by a reboot or power outage the next attempt to open the pool will need to complete the destroy operation synchronously.

When async_destroy is enabled the file system's data will be reclaimed by a background process, allowing the destroy operation to complete without traversing the entire file system. The background process is able to resume interrupted destroys after the pool has been opened, eliminating the need to finish interrupted destroys as part of the open operation. The amount of space remaining to be reclaimed by the background process is available through the freeing property.

This feature is only active while freeing is non-zero.

empty_bpobj

GUID	com.delphix:empty_bpobj
READ-ONLY COMPATIBLE	yes
DEPENDENCIES	none

This feature increases the performance of creating and using a large number of snapshots of a single filesystem or volume, and also reduces the disk space required.

When there are many snapshots, each snapshot uses many Block Pointer Objects (bpobj's) to track blocks associated with that snapshot. However, in common use cases, most of these bpobj's are empty. This feature allows us to create each bpobj on-demand, thus eliminating the empty bpobjs.

This feature is active while there are any filesystems, volumes, or snapshots which were created after enabling this feature.

filesystem_limits

GUID	com.joyent:filesystem_limits
READ-ONLY COMPATIBLE	yes
DEPENDENCIES	extensible_dataset

This feature enables filesystem and snapshot limits. These limits can be used to control how many filesystems and/or snapshots can be created at the point in the tree on which the limits are set.

This feature is active once either of the limit properties has been set on a dataset. Once activated the feature is never deactivated.

lz4_compress

GUID	org.illumos:lz4_compress
READ-ONLY COMPATIBLE	no
DEPENDENCIES	none

lz4 is a high-performance real-time compression algorithm that features significantly faster compression and decompression as well as a higher compression ratio than the older lzjb compression. Typically, lz4 compression is approximately 50% faster on compressible data and 200% faster on incompressible data than lzjb. It is also approximately 80% faster on decompression, while giving approximately 10% better compression ratio.

When the lz4_compress feature is set to enabled, the administrator can turn on lz4 compression on any dataset on the pool using the zfs(8) command. Please note that doing so will immediately activate the lz4_compress feature on the underlying pool using the zfs(1M) command. Also, all newly written metadata will be compressed with lz4 algorithm. Since this feature is not read-only compatible, this operation will render the pool unimportable on systems without support for the lz4_compress feature. Booting off of lz4-compressed root pools is supported.

This feature becomes active as soon as it is enabled and will never return to being enabled.

spacemap_histogram

GUID	com.delphix:spacemap_histogram
READ-ONLY COMPATIBLE	yes
DEPENDENCIES	none

This features allows ZFS to maintain more information about how free space is organized within the pool. If this feature is enabled, ZFS will set this feature to active when a new space map object is created or an existing space map is upgraded to the new format. Once the feature is active, it will remain in that state until the pool is destroyed.

extensible_dataset

GUID	com.delphix:extensible_dataset
READ-ONLY COMPATIBLE	no
DEPENDENCIES	none

This feature allows more flexible use of internal ZFS data structures, and exists for other features to depend on.

This feature will be active when the first dependent feature uses it, and will be returned to the enabled state when all datasets that use this feature are destroyed.

bookmarks

GUID	com.delphix:bookmarks
READ-ONLY COMPATIBLE	yes
DEPENDENCIES	extensible_dataset

This feature enables use of the zfs bookmark subcommand.

This feature is active while any bookmarks exist in the pool. All bookmarks in the pool can be listed by running zfs list -t bookmark -r poolname.

enabled_txg

GUID	com.delphix:enabled_txg
READ-ONLY COMPATIBLE	yes
DEPENDENCIES	none

Once this feature is enabled ZFS records the transaction group number in which new features are enabled. This has no user-visible impact, but other features may depend on this feature.

This feature becomes active as soon as it is enabled and will never return to being enabled.

hole_birth

GUID	com.delphix:hole_birth
READ-ONLY COMPATIBLE	no
DEPENDENCIES	enabled_txg

This feature improves performance of incremental sends ("zfs send -i") and receives for objects with many holes. The most common case of hole-filled objects is zvols.

An incremental send stream from snapshot A to snapshot B contains information about every block that changed between A and B. Blocks which did not change between those snapshots can be identified and omitted from the stream using a piece of metadata called the 'block birth time', but birth times are not recorded for holes (blocks filled only with zeroes). Since holes created after A cannot be distinguished from holes created before A, information about every hole in the entire filesystem or zvol is included in the send stream.

For workloads where holes are rare this is not a problem. However, when incrementally replicating filesystems or zvols with many holes (for example a zvol formatted with another filesystem) a lot of time will be spent sending and receiving unnecessary information about holes that already exist on the receiving side.

Once the hole_birth feature has been enabled the block birth times of all new holes will be recorded. Incremental sends between snapshots created after this feature is enabled will use this new metadata to avoid sending information about holes that already exist on the receiving side.

This feature becomes active as soon as it is enabled and will never return to being enabled.

embedded_data

GUID	com.delphix:embedded_data
READ-ONLY COMPATIBLE	no
DEPENDENCIES	none

This feature improves the performance and compression ratio of highly-compressible blocks. Blocks whose contents can compress to 112 bytes or smaller can take advantage of this feature.

When this feature is enabled, the contents of highly-compressible blocks are stored in the block "pointer" itself (a misnomer in this case, as it contains the compresseed data, rather than a pointer to its location on disk). Thus the space of the block (one sector, typically 512 bytes or 4KB) is saved, and no additional i/o is needed to read and write the data block.

This feature becomes active as soon as it is enabled and will never return to being enabled.

large_blocks

GUID	org.open-zfs:large_block
READ-ONLY COMPATIBLE	no
DEPENDENCIES	extensible_dataset

The large_block feature allows the record size on a dataset to be set larger than 128KB.

This feature becomes active once a recordsize property has been set larger than 128KB, and will return to being enabled once all filesystems that have ever had their recordsize larger than 128KB are destroyed.