ZPOOL-FEATURES(5)ZPOOL-FEATURES(5)NAMEzpool-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 for‐
mat number is 28). To enable a feature on a pool use the upgrade sub‐
command of the zpool(1M) command, or set the feature@feature_name prop‐
erty 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 soft‐
ware 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), how‐
ever 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 fea‐
ture to the active state. Some features may support return‐
ing to the enabled state after becoming active. See fea‐
ture-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 fea‐
ture 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 prop‐
erty named unsupported@feature_guid will indicate why the import was
allowed despite the unsupported feature. Possible values for this prop‐
erty 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 func‐
tion properly. Enabling a feature will automatically enable any fea‐
tures 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 com‐
plete 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 back‐
ground 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 remain‐
ing 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 fea‐
tures significantly faster compression and decompression as well as
a higher compression ratio than the older lzjb compression. Typi‐
cally, 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 approxi‐
mately 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. Please note that doing so will immediately acti‐
vate the lz4_compress feature on the underlying pool (even before
any data is written), and the feature will not be deactivated.
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.
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 com‐
prised of multiple vdevs. Those vdevs may be arranged in any mir‐
rored 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 struc‐
tures, 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 num‐
ber 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 snap‐
shots created after this feature is enabled will use this new meta‐
data 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.
SEE ALSOzpool(1M)
Aug 27, 2013 ZPOOL-FEATURES(5)
