Tribblix: 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.