ZPOOL-FEATURES(5) Standards, Environments, and Macros ZPOOL-FEATURES(5)


NAME


zpool-features - ZFS pool feature descriptions

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 upgrade subcommand
of the zpool(1M) command, 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
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.


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.
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(1M) for details
on importing pools).

Unsupported features


For each unsupported feature enabled on an imported pool a pool property
named unsupported@feature_guid 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
properly. Enabling a feature will automatically enable any features it
depends on.

FEATURES


The following features are supported on this system:

async_destroy

GUID com.delphix:async_destroy
READ-ONLY COMPATIBLE yes
DEPENDENCIES none

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.


empty_bpobj

GUID com.delphix:empty_bpobj
READ-ONLY COMPATIBLE yes
DEPENDENCIES none

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 active while there are any filesystems, volumes, or
snapshots which were created after enabling this feature.


filesystem_limits

GUID com.joyent:filesystem_limits
READ-ONLY COMPATIBLE yes
DEPENDENCIES extensible_dataset

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. Once activated the feature is never deactivated.


lz4_compress

GUID org.illumos:lz4_compress
READ-ONLY COMPATIBLE no
DEPENDENCIES none

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 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(1M) command. Also, all newly written metadata will be compressed
with lz4 algorithm. Since this feature is not read-only compatible,
this operation will render the pool unimportable on systems without
support for the lz4_compress feature. Booting off of lz4-compressed
root pools is supported.

This feature becomes active as soon as it is enabled and will never
return to being enabled.


spacemap_histogram

GUID com.delphix:spacemap_histogram
READ-ONLY COMPATIBLE yes
DEPENDENCIES none

This features allows ZFS to maintain more information about how free
space is organized within the pool. If this feature is enabled, ZFS
will set this feature to active when a new space map object is
created or an existing space map is upgraded to the new format. Once
the feature is active, it will remain in that state until the pool is
destroyed.


multi_vdev_crash_dump

GUID com.joyent:multi_vdev_crash_dump
READ-ONLY COMPATIBLE no
DEPENDENCIES none

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 the dumpadm(1M) command to configure a dump
device on a pool comprised of multiple vdevs.


extensible_dataset

GUID com.delphix:extensible_dataset
READ-ONLY COMPATIBLE no
DEPENDENCIES none

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.


bookmarks

GUID com.delphix:bookmarks
READ-ONLY COMPATIBLE yes
DEPENDENCIES extensible_dataset

This feature enables use of the zfs bookmark subcommand.

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.


enabled_txg

GUID com.delphix:enabled_txg
READ-ONLY COMPATIBLE yes
DEPENDENCIES none

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.


hole_birth

GUID com.delphix:hole_birth
READ-ONLY COMPATIBLE no
DEPENDENCIES enabled_txg

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.


embedded_data

GUID com.delphix:embedded_data
READ-ONLY COMPATIBLE no
DEPENDENCIES none

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 active as soon as it is enabled and will never
return to being enabled.


device_removal

GUID com.delphix:device_removal
READ-ONLY COMPATIBLE no
DEPENDENCIES none

This feature enables the "zpool remove" subcommand 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.


obsolete_counts

GUID com.delphix:obsolete_counts
READ-ONLY COMPATIBLE yes
DEPENDENCIES device_removal

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 zfs(1M).

This feature becomes active when the "zpool remove" command is used
on a top-level vdev, and will never return to being enabled.


large_blocks

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

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

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


sha512

GUID org.illumos:sha512
READ-ONLY COMPATIBLE no
DEPENDENCIES extensible_dataset

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 the zfs set
checksum=sha512 command. 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.

Booting off of pools utilizing SHA-512/256 is supported (provided
that the updated GRUB stage2 module is installed).


skein

GUID org.illumos:skein
READ-ONLY COMPATIBLE no
DEPENDENCIES extensible_dataset

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 the zfs set checksum=skein
command. 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.

Booting off of pools using skein is NOT supported -- any attempt to
enable skein on a root pool will fail with an error.


edonr

GUID org.illumos:edonr
READ-ONLY COMPATIBLE no
DEPENDENCIES extensible_dataset

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 can not be used
with dedup (without verification).

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.

When the edonr feature is set to enabled, the administrator can turn
on the edonr checksum on any dataset using the zfs set checksum=edonr
command. 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.

Booting off of pools using edonr is NOT supported -- any attempt to
enable edonr on a root pool will fail with an error.


SEE ALSO


zfs(1M), zpool(1M)


August 27, 2013 ZPOOL-FEATURES(5)