'\" te
.\" Copyright (c) 2013 by Delphix. All rights reserved.
.\" Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
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.TH ZPOOL-FEATURES 5 "Aug 27, 2013"
.SH NAME
zpool\-features \- ZFS pool feature descriptions
.SH DESCRIPTION
.sp
.LP
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 \fBupgrade\fR subcommand of the
\fBzpool\fR(8) command, or set the \fBfeature@\fR\fIfeature_name\fR property
to \fBenabled\fR.
.sp
.LP
The pool format does not affect file system version compatibility or the ability
to send file systems between pools.
.sp
.LP
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
\fBactive\fR on the pool. If the pool was created by another software version
this set may include unsupported features.
.SS "Identifying features"
.sp
.LP
Every feature has a guid of the form \fIcom.example:feature_name\fR. 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.
.sp
.LP
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.
\fIcom.example:feature_name\fR would have the short name \fIfeature_name\fR),
however a feature's short name may differ across ZFS implementations if
following the convention would result in name conflicts.
.SS "Feature states"
.sp
.LP
Features can be in one of three states:
.sp
.ne 2
.na
\fB\fBactive\fR\fR
.ad
.RS 12n
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").
.RE

.sp
.ne 2
.na
\fB\fBenabled\fR\fR
.ad
.RS 12n
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
\fBactive\fR state. Some features may support returning to the \fBenabled\fR
state after becoming \fBactive\fR. See feature\-specific documentation for
details.
.RE

.sp
.ne 2
.na
\fBdisabled\fR
.ad
.RS 12n
This feature's on\-disk format changes have not been made and will not be made
unless an administrator moves the feature to the \fBenabled\fR state. Features
cannot be disabled once they have been enabled.
.RE

.sp
.LP
The state of supported features is exposed through pool properties of the form
\fIfeature@short_name\fR.
.SS "Read\-only compatibility"
.sp
.LP
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
\fBreadonly\fR property during import (see \fBzpool\fR(8) for details on
importing pools).
.SS "Unsupported features"
.sp
.LP
For each unsupported feature enabled on an imported pool a pool property
named \fIunsupported@feature_guid\fR will indicate why the import was allowed
despite the unsupported feature. Possible values for this property are:

.sp
.ne 2
.na
\fB\fBinactive\fR\fR
.ad
.RS 12n
The feature is in the \fBenabled\fR state and therefore the pool's on\-disk
format is still compatible with software that does not support this feature.
.RE

.sp
.ne 2
.na
\fB\fBreadonly\fR\fR
.ad
.RS 12n
The feature is read\-only compatible and the pool has been imported in
read\-only mode.
.RE

.SS "Feature dependencies"
.sp
.LP
Some features depend on other features being enabled in order to function
properly. Enabling a feature will automatically enable any features it
depends on.
.SH FEATURES
.sp
.LP
The following features are supported on this system:
.sp
.ne 2
.na
\fB\fBasync_destroy\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	com.delphix:async_destroy
READ\-ONLY COMPATIBLE	yes
DEPENDENCIES	none
.TE

Destroying a file system requires traversing all of its data in order to
return its used space to the pool. Without \fBasync_destroy\fR 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 \fBasync_destroy\fR 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 \fBfreeing\fR property.

This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero.
.RE

.sp
.ne 2
.na
\fB\fBempty_bpobj\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	com.delphix:empty_bpobj
READ\-ONLY COMPATIBLE	yes
DEPENDENCIES	none
.TE

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 \fBactive\fR while there are any filesystems, volumes,
or snapshots which were created after enabling this feature.
.RE

.sp
.ne 2
.na
\fB\fBlz4_compress\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	org.illumos:lz4_compress
READ\-ONLY COMPATIBLE	no
DEPENDENCIES	none
.TE

\fBlz4\fR 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 \fBlzjb\fR compression.
Typically, \fBlz4\fR compression is approximately 50% faster on
compressible data and 200% faster on incompressible data than
\fBlzjb\fR. It is also approximately 80% faster on decompression, while
giving approximately 10% better compression ratio.

When the \fBlz4_compress\fR feature is set to \fBenabled\fR, the
administrator can turn on \fBlz4\fR compression on any dataset on the
pool using the \fBzfs\fR(8) command. Please note that doing so will
immediately activate the \fBlz4_compress\fR feature on the underlying
pool (even before any data is written). Since this feature is not
read-only compatible, this operation will render the pool unimportable
on systems without support for the \fBlz4_compress\fR feature. At the
moment, this operation cannot be reversed. Booting off of
\fBlz4\fR-compressed root pools is supported.
.RE

.sp
.ne 2
.na
\fB\fBspacemap_histogram\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	com.delphix:spacemap_histogram
READ\-ONLY COMPATIBLE	yes
DEPENDENCIES	none
.TE

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

.RE

.sp
.ne 2
.na
\fB\fBextensible_dataset\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	com.delphix:extensible_dataset
READ\-ONLY COMPATIBLE	no
DEPENDENCIES	none
.TE

This feature allows more flexible use of internal ZFS data structures,
and exists for other features to depend on.

This feature will be \fBactive\fR when the first dependent feature uses it,
and will be returned to the \fBenabled\fR state when all datasets that use
this feature are destroyed.

.RE

.sp
.ne 2
.na
\fB\fBenabled_txg\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	com.delphix:enabled_txg
READ\-ONLY COMPATIBLE	yes
DEPENDENCIES	none
.TE

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 \fBactive\fR as soon as it is enabled and will
never return to being \fBenabled\fB.

.RE

.sp
.ne 2
.na
\fB\fBhole_birth\fR\fR
.ad
.RS 4n
.TS
l l .
GUID	com.delphix:hole_birth
READ\-ONLY COMPATIBLE	no
DEPENDENCIES	enabled_txg
.TE

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 \fBA\fR to snapshot \fBB\fR
contains information about every block that changed between \fBA\fR and
\fBB\fR. 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 \fBA\fR cannot be
distinguished from holes created before \fBA\fR, 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 \fBhole_birth\fR 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 \fBactive\fR as soon as it is enabled and will
never return to being \fBenabled\fB.

.RE


.SH "SEE ALSO"
\fBzpool\fR(8)