te Copyright (c) 2013 by Delphix. All rights reserved. Copyright (c) 2013 by Saso Kiselkov. All rights reserved. Copyright (c) 2014, Joyent, Inc. All rights reserved. The contents of this file are subject to the terms of the Common Development and Distribution License (the "License"). You may not use this file except in compliance with the License. You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE or http://www.opensolaris.org/os/licensing. See the License for the specific language governing permissions and limitations under the License. When distributing Covered Code, include this CDDL HEADER in each file and include the License file at usr/src/OPENSOLARIS.LICENSE. If applicable, add the following below this CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner] ZPOOL-FEATURES 5 "Aug 27, 2013"
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").
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.
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.
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.
|
|
| 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.
|
|
| 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.
|
|
| 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. Please note that doing so will
immediately activate 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
|
|
| 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.
"SEE ALSO"
zpool(1M)