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If applicable, add the following below this .\" CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your .\" own identifying information: .\" Portions Copyright [yyyy] [name of copyright owner] .TH ZPOOL-FEATURES 5 "Aug 27, 2013" .SH NAME zpool\-features \- ZFS pool feature descriptions .SH DESCRIPTION .sp .LP 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 \fBupgrade\fR subcommand of the \fBzpool\fR(8) command, or set the \fBfeature@\fR\fIfeature_name\fR property to \fBenabled\fR. .sp .LP The pool format does not affect file system version compatibility or the ability to send file systems between pools. .sp .LP 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 \fBactive\fR on the pool. If the pool was created by another software version this set may include unsupported features. .SS "Identifying features" .sp .LP Every feature has a guid of the form \fIcom.example:feature_name\fR. 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. .sp .LP 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. \fIcom.example:feature_name\fR would have the short name \fIfeature_name\fR), however a feature's short name may differ across ZFS implementations if following the convention would result in name conflicts. .SS "Feature states" .sp .LP Features can be in one of three states: .sp .ne 2 .na \fB\fBactive\fR\fR .ad .RS 12n 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"). .RE .sp .ne 2 .na \fB\fBenabled\fR\fR .ad .RS 12n 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 \fBactive\fR state. Some features may support returning to the \fBenabled\fR state after becoming \fBactive\fR. See feature\-specific documentation for details. .RE .sp .ne 2 .na \fBdisabled\fR .ad .RS 12n This feature's on\-disk format changes have not been made and will not be made unless an administrator moves the feature to the \fBenabled\fR state. Features cannot be disabled once they have been enabled. .RE .sp .LP The state of supported features is exposed through pool properties of the form \fIfeature@short_name\fR. .SS "Read\-only compatibility" .sp .LP 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 \fBreadonly\fR property during import (see \fBzpool\fR(8) for details on importing pools). .SS "Unsupported features" .sp .LP For each unsupported feature enabled on an imported pool a pool property named \fIunsupported@feature_guid\fR will indicate why the import was allowed despite the unsupported feature. Possible values for this property are: .sp .ne 2 .na \fB\fBinactive\fR\fR .ad .RS 12n The feature is in the \fBenabled\fR state and therefore the pool's on\-disk format is still compatible with software that does not support this feature. .RE .sp .ne 2 .na \fB\fBreadonly\fR\fR .ad .RS 12n The feature is read\-only compatible and the pool has been imported in read\-only mode. .RE .SS "Feature dependencies" .sp .LP 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 .sp .LP The following features are supported on this system: .sp .ne 2 .na \fB\fBasync_destroy\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:async_destroy READ\-ONLY COMPATIBLE yes DEPENDENCIES none .TE Destroying a file system requires traversing all of its data in order to return its used space to the pool. Without \fBasync_destroy\fR 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 \fBasync_destroy\fR 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 \fBfreeing\fR property. This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero. .RE .sp .ne 2 .na \fB\fBempty_bpobj\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:empty_bpobj READ\-ONLY COMPATIBLE yes DEPENDENCIES none .TE 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 \fBactive\fR while there are any filesystems, volumes, or snapshots which were created after enabling this feature. .RE .sp .ne 2 .na \fB\fBlz4_compress\fR\fR .ad .RS 4n .TS l l . GUID org.illumos:lz4_compress READ\-ONLY COMPATIBLE no DEPENDENCIES none .TE \fBlz4\fR 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 \fBlzjb\fR compression. Typically, \fBlz4\fR compression is approximately 50% faster on compressible data and 200% faster on incompressible data than \fBlzjb\fR. It is also approximately 80% faster on decompression, while giving approximately 10% better compression ratio. When the \fBlz4_compress\fR feature is set to \fBenabled\fR, the administrator can turn on \fBlz4\fR compression on any dataset on the pool using the \fBzfs\fR(8) command. Please note that doing so will immediately activate the \fBlz4_compress\fR feature on the underlying pool using the \fBzfs\fR(1M) command. Also, all newly written metadata will be compressed with \fBlz4\fR algorithm. Since this feature is not read-only compatible, this operation will render the pool unimportable on systems without support for the \fBlz4_compress\fR feature. Booting off of \fBlz4\fR-compressed root pools is supported. This feature becomes \fBactive\fR as soon as it is enabled and will never return to being \fBenabled\fB. .RE .sp .ne 2 .na \fB\fBspacemap_histogram\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:spacemap_histogram READ\-ONLY COMPATIBLE yes DEPENDENCIES none .TE This features allows ZFS to maintain more information about how free space is organized within the pool. If this feature is \fBenabled\fR, ZFS will set this feature to \fBactive\fR when a new space map object is created or an existing space map is upgraded to the new format. Once the feature is \fBactive\fR, it will remain in that state until the pool is destroyed. .RE .sp .ne 2 .na \fB\fBextensible_dataset\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:extensible_dataset READ\-ONLY COMPATIBLE no DEPENDENCIES none .TE This feature allows more flexible use of internal ZFS data structures, and exists for other features to depend on. This feature will be \fBactive\fR when the first dependent feature uses it, and will be returned to the \fBenabled\fR state when all datasets that use this feature are destroyed. .RE .sp .ne 2 .na \fB\fBbookmarks\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:bookmarks READ\-ONLY COMPATIBLE yes DEPENDENCIES extensible_dataset .TE This feature enables use of the \fBzfs bookmark\fR subcommand. This feature is \fBactive\fR while any bookmarks exist in the pool. All bookmarks in the pool can be listed by running \fBzfs list -t bookmark -r \fIpoolname\fR\fR. .RE .sp .ne 2 .na \fB\fBenabled_txg\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:enabled_txg READ\-ONLY COMPATIBLE yes DEPENDENCIES none .TE 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 \fBactive\fR as soon as it is enabled and will never return to being \fBenabled\fB. .RE .sp .ne 2 .na \fB\fBhole_birth\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:hole_birth READ\-ONLY COMPATIBLE no DEPENDENCIES enabled_txg .TE 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 \fBA\fR to snapshot \fBB\fR contains information about every block that changed between \fBA\fR and \fBB\fR. 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 \fBA\fR cannot be distinguished from holes created before \fBA\fR, 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 \fBhole_birth\fR 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 \fBactive\fR as soon as it is enabled and will never return to being \fBenabled\fB. .RE .sp .ne 2 .na \fB\fBembedded_data\fR\fR .ad .RS 4n .TS l l . GUID com.delphix:embedded_data READ\-ONLY COMPATIBLE no DEPENDENCIES none .TE 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 \fBactive\fR as soon as it is enabled and will never return to being \fBenabled\fR. .RE .SH "SEE ALSO" \fBzpool\fR(8)