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(8) 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(8) 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(8) command. Please note that doing so will immediately activate the
lz4_compress feature on the underlying 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.
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.
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.
SEE ALSO¶
zpool(8)
