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Copyright (c) 2012, Martin Matuska <mm@FreeBSD.org>.
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Copyright (c) 2012 by Delphix. All rights reserved.
Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
Copyright (c) 2013, Joyent, Inc. All rights reserved.

$FreeBSD: stable/10/cddl/contrib/opensolaris/cmd/zpool/zpool-features.7 276081 2014-12-22 20:58:51Z delphij $

.Dd November 10, 2014 .Dt ZPOOL-FEATURES 7 .Os .Sh NAME .Nm zpool-features .Nd ZFS pool feature descriptions .Sh 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 .Cm upgrade subcommand of the .Xr zpool 8 command, or set the .Sy feature@feature_name property to .Ar enabled .

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

p 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 .Sy active on the pool. If the pool was created by another software version this set may include unsupported features. .Ss Identifying features Every feature has a guid of the form .Sy 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.

p 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. .Sy com.example:feature_name would have the short name .Sy feature_name ), however a feature's short name may differ across ZFS implementations if following the convention would result in name conflicts. .Ss Feature states Features can be in one of three states: l -tag -width "XXXXXXXX" t Sy 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"). t Sy 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 .Sy active state. Some features may support returning to the .Sy enabled state after becoming .Sy active . See feature-specific documentation for details. t Sy 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 .Sy enabled state. Features cannot be disabled once they have been enabled. .El

p The state of supported features is exposed through pool properties of the form .Sy feature@short_name . .Ss 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 .Sy readonly property during import (see .Xr zpool 8 for details on importing pools). .Ss Unsupported features For each unsupported feature enabled on an imported pool a pool property named .Sy unsupported@feature_guid will indicate why the import was allowed despite the unsupported feature. Possible values for this property are: l -tag -width "XXXXXXXX" t Sy inactive The feature is in the .Sy enabled state and therefore the pool's on-disk format is still compatible with software that does not support this feature. t Sy readonly The feature is read-only compatible and the pool has been imported in read-only mode. .El .Ss 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. .Sh FEATURES The following features are supported on this system: l -tag -width "XXXXXXXX" t Sy async_destroy l -column "READ-ONLY COMPATIBLE" "com.delphix:async_destroy" t GUID Ta com.delphix:async_destroy t READ-ONLY COMPATIBLE Ta yes t DEPENDENCIES Ta none .El

p Destroying a file system requires traversing all of its data in order to return its used space to the pool. Without .Sy 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.

p When .Sy 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 .Sy freeing property.

p This feature is only .Sy active while .Sy freeing is non-zero. t Sy empty_bpobj l -column "READ-ONLY COMPATIBLE" "com.delphix:empty_bpobj" t GUID Ta com.delphix:empty_bpobj t READ-ONLY COMPATIBLE Ta yes t DEPENDENCIES Ta none .El

p 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.

p When there are many snapshots, each snapshot uses many Block Pointer Objects

q 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.

p This feature is .Sy active while there are any filesystems, volumes, or snapshots which were created after enabling this feature. t Sy filesystem_limits l -column "READ-ONLY COMPATIBLE" "com.joyent:filesystem_limits" t GUID Ta com.joyent:filesystem_limits t READ-ONLY COMPATIBLE Ta yes t DEPENDENCIES Ta extensible_dataset .El

p 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.

p This feature is .Sy active once either of the limit properties has been set on a dataset. Once activated the feature is never deactivated. t Sy lz4_compress l -column "READ-ONLY COMPATIBLE" "org.illumos:lz4_compress" t GUID Ta org.illumos:lz4_compress t READ-ONLY COMPATIBLE Ta no t DEPENDENCIES Ta none .El

p .Sy 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 .Sy lzjb compression. Typically, .Sy lz4 compression is approximately 50% faster on compressible data and 200% faster on incompressible data than .Sy lzjb . It is also approximately 80% faster on decompression, while giving approximately 10% better compression ratio.

p When the .Sy lz4_compress feature is set to .Sy enabled , the administrator can turn on .Sy lz4 compression on any dataset on the pool using the .Xr zfs 8 command. Also, all newly written metadata will be compressed with .Sy lz4 algorithm. Since this feature is not read-only compatible, this operation will render the pool unimportable on systems without support for the .Sy lz4_compress feature. Booting off of .Sy lz4 -compressed root pools is supported.

p This feature becomes .Sy active as soon as it is enabled and will never return to being .Sy enabled . t Sy multi_vdev_crash_dump l -column "READ-ONLY COMPATIBLE" "com.joyent:multi_vdev_crash_dump" t GUID Ta com.joyent:multi_vdev_crash_dump t READ-ONLY COMPATIBLE Ta no t DEPENDENCIES Ta none .El

p 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. TODO: this is not yet supported on FreeBSD.
.Pp
When the
.Sy multi_vdev_crash_dump
feature is set to
.Sy enabled ,
the administrator can use the
.Xr dumpon 8
command to configure a
dump device on a pool comprised of multiple vdevs.
t Sy spacemap_histogram l -column "READ-ONLY COMPATIBLE" "com.delphix:spacemap_histogram" t GUID Ta com.delphix:spacemap_histogram t READ-ONLY COMPATIBLE Ta yes t DEPENDENCIES Ta none .El

p This features allows ZFS to maintain more information about how free space is organized within the pool. If this feature is .Sy enabled , ZFS will set this feature to .Sy active when a new space map object is created or an existing space map is upgraded to the new format. Once the feature is .Sy active , it will remain in that state until the pool is destroyed. t Sy extensible_dataset l -column "READ-ONLY COMPATIBLE" "com.delphix:extensible_dataset" t GUID Ta com.delphix:extensible_dataset t READ-ONLY COMPATIBLE Ta no t DEPENDENCIES Ta none .El

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

p This feature will be .Sy active when the first dependent feature uses it, and will be returned to the .Sy enabled state when all datasets that use this feature are destroyed. t Sy bookmarks l -column "READ-ONLY COMPATIBLE" "com.delphix:bookmarks" t GUID Ta com.delphix:bookmarks t READ-ONLY COMPATIBLE Ta yes t DEPENDENCIES Ta extensible_dataset .El

p This feature enables use of the .Nm zfs .Cm bookmark subcommand.

p This feature is .Sy active while any bookmarks exist in the pool. All bookmarks in the pool can be listed by running .Nm zfs .Cm list .Fl t No bookmark Fl r Ar poolname . t Sy enabled_txg l -column "READ-ONLY COMPATIBLE" "com.delphix:enabled_txg" t GUID Ta com.delphix:enabled_txg t READ-ONLY COMPATIBLE Ta yes t DEPENDENCIES Ta none .El

p 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.

p This feature becomes .Sy active as soon as it is enabled and will never return to being .Sy enabled . t Sy hole_birth l -column "READ-ONLY COMPATIBLE" "com.delphix:hole_birth" t GUID Ta com.delphix:hole_birth t READ-ONLY COMPATIBLE Ta no t DEPENDENCIES Ta enabled_txg .El

p This feature improves performance of incremental sends

q Dq zfs send -i and receives for objects with many holes. The most common case of hole-filled objects is zvols.

p An incremental send stream from snapshot .Sy A to snapshot .Sy B contains information about every block that changed between .Sy A and .Sy 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

q blocks filled only with zeroes . Since holes created after .Sy A cannot be distinguished from holes created before .Sy A , information about every hole in the entire filesystem or zvol is included in the send stream.

p For workloads where holes are rare this is not a problem. However, when incrementally replicating filesystems or zvols with many holes

q 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.

p Once the .Sy 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.

p This feature becomes .Sy active as soon as it is enabled and will never return to being .Sy enabled . t Sy embedded_data l -column "READ-ONLY COMPATIBLE" "com.delphix:embedded_data" t GUID Ta com.delphix:embedded_data t READ-ONLY COMPATIBLE Ta no t DEPENDENCIES Ta none .El

p 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.

p When this feature is enabled, the contents of highly-compressible blocks are stored in the block "pointer" itself

o a misnomer in this case, as it contains the compressed data, rather than a pointer to its location on disk

c . Thus the space of the block

q one sector, typically 512 bytes or 4KB is saved, and no additional i/o is needed to read and write the data block.

p This feature becomes .Sy active as soon as it is enabled and will never return to being .Sy enabled . t Sy large_blocks l -column "READ-ONLY COMPATIBLE" "org.open-zfs:large_block" t GUID Ta org.open-zfs:large_block t READ-ONLY COMPATIBLE Ta no t DEPENDENCIES Ta extensible_dataset .El

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

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

p Please note that booting from datasets that have recordsize greater than 128KB is .Em NOT supported by the .Fx boot loader. .El .Sh SEE ALSO .Xr zpool 8 .Sh AUTHORS This manual page is a .Xr mdoc 7 reimplementation of the .Tn illumos manual page .Em zpool-features(5) , modified and customized for .Fx and licensed under the Common Development and Distribution License

q Tn CDDL .

p The .Xr mdoc 7 implementation of this manual page was initially written by .An Martin Matuska Aq mm@FreeBSD.org .