'\" te
.\" Copyright (c) 2012, Martin Matuska <mm@FreeBSD.org>.
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.\" Copyright (c) 2012, 2017 by Delphix. All rights reserved.
.\" Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
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.Dd June 7, 2017
.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 .
.Pp
The pool format does not affect file system version compatibility or the ability
to send file systems between pools.
.Pp
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.
.Pp
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:
.Bl -tag -width "XXXXXXXX"
.It 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").
.It 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.
.It 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
.Pp
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:
.Bl -tag -width "XXXXXXXX"
.It 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.
.It 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:
.Bl -tag -width "XXXXXXXX"
.It Sy async_destroy
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:async_destroy"
.It GUID Ta com.delphix:async_destroy
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
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.
.Pp
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.
.Pp
This feature is only
.Sy active
while
.Sy freeing
is non\-zero.
.It Sy empty_bpobj
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:empty_bpobj"
.It GUID Ta com.delphix:empty_bpobj
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
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.
.Pp
When there are many snapshots, each snapshot uses many Block Pointer Objects
.Pq 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.
.Pp
This feature is
.Sy active
while there are any filesystems, volumes, or snapshots which were created
after enabling this feature.
.It Sy filesystem_limits
.Bl -column "READ\-ONLY COMPATIBLE" "com.joyent:filesystem_limits"
.It GUID Ta com.joyent:filesystem_limits
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta extensible_dataset
.El
.Pp
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.
.Pp
This feature is
.Sy active
once either of the limit properties has been
set on a dataset.
Once activated the feature is never deactivated.
.It Sy lz4_compress
.Bl -column "READ\-ONLY COMPATIBLE" "org.illumos:lz4_compress"
.It GUID Ta org.illumos:lz4_compress
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
.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.
.Pp
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.
.Pp
This feature becomes
.Sy active
as soon as it is enabled and will
never return to being
.Sy enabled .
.It Sy multi_vdev_crash_dump
.Bl -column "READ\-ONLY COMPATIBLE" "com.joyent:multi_vdev_crash_dump"
.It GUID Ta com.joyent:multi_vdev_crash_dump
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
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.
.It Sy spacemap_histogram
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:spacemap_histogram"
.It GUID Ta com.delphix:spacemap_histogram
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
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.
.It Sy extensible_dataset
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:extensible_dataset"
.It GUID Ta com.delphix:extensible_dataset
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
This feature allows more flexible use of internal ZFS data structures,
and exists for other features to depend on.
.Pp
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.
.It Sy bookmarks
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:bookmarks"
.It GUID Ta com.delphix:bookmarks
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta extensible_dataset
.El
.Pp
This feature enables use of the
.Nm zfs
.Cm bookmark
subcommand.
.Pp
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 .
.It Sy enabled_txg
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:enabled_txg"
.It GUID Ta com.delphix:enabled_txg
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
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.
.Pp
This feature becomes
.Sy active
as soon as it is enabled and will
never return to being
.Sy enabled .
.It Sy hole_birth
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:hole_birth"
.It GUID Ta com.delphix:hole_birth
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta enabled_txg
.El
.Pp
This feature improves performance of incremental sends
.Pq Dq zfs send -i
and receives for objects with many holes.
The most common case of
hole-filled objects is zvols.
.Pp
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
.Pq 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.
.Pp
For workloads where holes are rare this is not a problem.
However, when
incrementally replicating filesystems or zvols with many holes
.Pq 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.
.Pp
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.
.Pp
This feature becomes
.Sy active
as soon as it is enabled and will
never return to being
.Sy enabled .
.It Sy embedded_data
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:embedded_data"
.It GUID Ta com.delphix:embedded_data
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
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.
.Pp
When this feature is enabled, the contents of highly-compressible blocks are
stored in the block "pointer" itself
.Po a misnomer in this case, as it contains
the compressed data, rather than a pointer to its location on disk
.Pc .
Thus
the space of the block
.Pq one sector, typically 512 bytes or 4KB
is saved,
and no additional i/o is needed to read and write the data block.
.Pp
This feature becomes
.Sy active
as soon as it is enabled and will
never return to being
.Sy enabled .
.It Sy zpool_checkpoint
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:zpool_checkpoint"
.It GUID Ta com.delphix:zpool_checkpoint
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
This feature enables the "zpool checkpoint" subcommand that can
checkpoint the state of the pool at the time it was issued and later
rewind back to it or discard it.
.Pp
This feature becomes
.Sy active
when the "zpool checkpoint" command is used to checkpoint the pool.
The feature will only return back to being
.Sy enabled
when the pool is rewound or the checkpoint has been discarded.
.It Sy device_removal
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:device_removal"
.It GUID Ta com.delphix:device_removal
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
This feature enables the "zpool remove" subcommand to remove top-level
vdevs, evacuating them to reduce the total size of the pool.
.Pp
This feature becomes
.Sy active
when the "zpool remove" command is used
on a top-level vdev, and will never return to being 
.Sy enabled .
.It Sy obsolete_counts
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:obsolete_counts"
.It GUID Ta com.delphix:obsolete_counts
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta device_removal
.El
.Pp
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", i.e. no longer needed.  See also the "zfs remap"
subcommand in
.Xr zfs 8 .

This feature becomes
.Sy active
when the "zpool remove" command is
used on a top-level vdev, and will never return to being
.Sy enabled .
.It Sy spacemap_v2
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:spacemap_v2"
.It GUID Ta com.delphix:spacemap_v2
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
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.
.Pp
This feature becomes
.Sy active
as soon as it is enabled and will
never return to being
.Sy enabled .
.It Sy large_blocks
.Bl -column "READ\-ONLY COMPATIBLE" "org.open-zfs:large_block"
.It GUID Ta org.open-zfs:large_block
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta extensible_dataset
.El
.Pp
The
.Sy large_block
feature allows the record size on a dataset to be
set larger than 128KB.
.Pp
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.
.Pp
Please note that booting from datasets that have recordsize greater than
128KB is
.Em NOT
supported by the
.Fx
boot loader.
.It Sy sha512
.Bl -column "READ\-ONLY COMPATIBLE" "org.illumos:sha512"
.It GUID Ta org.illumos:sha512
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
The
.Sy sha512
feature enables the use of the SHA-512/256 truncated hash algorithm
.Pq 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
.Sy skein
algorithms.
.Pp
When the
.Sy sha512
feature is set to
.Sy enabled ,
the administrator can turn on the
.Sy sha512
checksum on any dataset using the
.Dl # zfs set checksum=sha512 Ar dataset
command.
This feature becomes
.Sy active
once a
.Sy checksum
property has been set to
.Sy sha512 ,
and will return to being
.Sy enabled
once all filesystems that have ever had their checksum set to
.Sy sha512
are destroyed.
.It Sy skein
.Bl -column "READ\-ONLY COMPATIBLE" "org.illumos:skein"
.It GUID Ta org.illumos:skein
.It READ\-ONLY COMPATIBLE Ta no
.It DEPENDENCIES Ta none
.El
.Pp
The
.Sy skein
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
.Pq 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
.Pq 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.
.Pp
When the
.Sy skein
feature is set to
.Sy enabled ,
the administrator can turn on the
.Sy skein
checksum on any dataset using the
.Dl # zfs set checksum=skein Ar dataset
command.
This feature becomes
.Sy active
once a
.Sy checksum
property has been set to
.Sy skein ,
and will return to being
.Sy enabled
once all filesystems that have ever had their checksum set to
.Sy skein
are destroyed.
.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
.Pq Tn CDDL .
.Pp
The
.Xr mdoc 7
implementation of this manual page was initially written by
.An Martin Matuska Aq mm@FreeBSD.org .