illumos: manual page: zpool-features.7

ZPOOL-FEATURES(7) Standards, Environments, and Macros 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.

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:

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-name 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.
Enabling a feature will automatically enable any features it depends on.

FEATURES

The following features are supported on this system:

allocation_classes
GUID org.zfsonlinux:allocation_classes
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.

async_destroy
GUID com.delphix:async_destroy
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.

bookmarks
GUID com.delphix:bookmarks
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 bookmark
-r poolname.

bookmark_v2
GUID com.datto:bookmark_v2
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.

device_removal
GUID com.delphix:device_removal
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.

edonr
GUID org.illumos:edonr
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
dedup=edonr,verify (see zfs(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(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.

embedded_data
GUID com.delphix:embedded_data
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 512 B or 4 KiB) 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.

empty_bpobj
GUID com.delphix:empty_bpobj
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.

enabled_txg
GUID com.delphix:enabled_txg
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.

encryption
GUID com.datto:encryption
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.

extensible_dataset
GUID com.delphix:extensible_dataset
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.

filesystem_limits
GUID com.joyent:filesystem_limits
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 and will never return to being enabled.

hole_birth
GUID com.delphix:hole_birth
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
send_holes_without_birth_time 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.

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

This feature allows the record size on a dataset to be set larger
than 128 KiB.

This feature becomes active once a dataset contains a file with a
block size larger than 128 KiB, and will return to being enabled
once all filesystems that have ever had their recordsize larger
than 128 KiB are destroyed.

large_dnode
GUID org.zfsonlinux:large_dnode
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE no

This feature allows the size of dnodes in a dataset to be set
larger than 512 B. This feature becomes active once a dataset
contains an object with a dnode larger than 512 B, which occurs as
a result of setting the dnodesize 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 512 B 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.

log_spacemap
GUID com.delphix:log_spacemap
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.

lz4_compress
GUID org.illumos:lz4_compress
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(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.

multi_vdev_crash_dump
GUID com.joyent:multi_vdev_crash_dump
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(8) 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.

obsolete_counts
GUID com.delphix:obsolete_counts
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.

project_quota
GUID org.zfsonlinux:project_quota
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 [+-]P 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.

resilver_defer
GUID com.datto:resilver_defer
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.

sha512
GUID org.illumos:sha512
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(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.

skein
GUID org.illumos:skein
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(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.

spacemap_histogram
GUID com.delphix:spacemap_histogram
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.

spacemap_v2
GUID com.delphix:spacemap_v2
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.

userobj_accounting
GUID org.zfsonlinux:userobj_accounting
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.

zilsaxattr
GUID org.openzfs:zilsaxattr
DEPENDENCIES extensible_dataset
READ-ONLY COMPATIBLE yes

This feature enables xattr=sa extended attribute logging in the
ZIL. If enabled, extended attribute changes (both xattrdir=dir and
xattr=sa) are guaranteed to be durable if either the dataset had
sync=always set at the time the changes were made, or sync(2) is
called on the dataset after the changes were made.

This feature becomes active when a ZIL is created for at least one
dataset and will be returned to the enabled state when it is
destroyed for all datasets that use this feature.

zpool_checkpoint
GUID com.delphix:zpool_checkpoint
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.