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  17 .TH ZPOOL-FEATURES 5 "Aug 27, 2013"
  18 .SH NAME
  19 zpool\-features \- ZFS pool feature descriptions
  21 .sp
  22 .LP
  23 ZFS pool on\-disk format versions are specified via "features" which replace
  24 the old on\-disk format numbers (the last supported on\-disk format number is
  25 28). To enable a feature on a pool use the \fBupgrade\fR subcommand of the
  26 \fBzpool\fR(1M) command, or set the \fBfeature@\fR\fIfeature_name\fR property
  27 to \fBenabled\fR.
  28 .sp
  29 .LP
  30 The pool format does not affect file system version compatibility or the ability
  31 to send file systems between pools.
  32 .sp
  33 .LP
  34 Since most features can be enabled independently of each other the on\-disk
  35 format of the pool is specified by the set of all features marked as
  36 \fBactive\fR on the pool. If the pool was created by another software version
  37 this set may include unsupported features.
  38 .SS "Identifying features"
  39 .sp
  40 .LP
  41 Every feature has a guid of the form \fIcom.example:feature_name\fR. The reverse
  42 DNS name ensures that the feature's guid is unique across all ZFS
  43 implementations. When unsupported features are encountered on a pool they will
  44 be identified by their guids. Refer to the documentation for the ZFS
  45 implementation that created the pool for information about those features.
  46 .sp
  47 .LP
  48 Each supported feature also has a short name. By convention a feature's short
  49 name is the portion of its guid which follows the ':' (e.g.
  50 \fIcom.example:feature_name\fR would have the short name \fIfeature_name\fR),
  51 however a feature's short name may differ across ZFS implementations if
  52 following the convention would result in name conflicts.
  53 .SS "Feature states"
  54 .sp
  55 .LP
  56 Features can be in one of three states:
  57 .sp
  58 .ne 2
  59 .na
  60 \fB\fBactive\fR\fR
  61 .ad
  62 .RS 12n
  63 This feature's on\-disk format changes are in effect on the pool. Support for
  64 this feature is required to import the pool in read\-write mode. If this
  65 feature is not read-only compatible, support is also required to import the pool
  66 in read\-only mode (see "Read\-only compatibility").
  67 .RE
  69 .sp
  70 .ne 2
  71 .na
  72 \fB\fBenabled\fR\fR
  73 .ad
  74 .RS 12n
  75 An administrator has marked this feature as enabled on the pool, but the
  76 feature's on\-disk format changes have not been made yet. The pool can still be
  77 imported by software that does not support this feature, but changes may be made
  78 to the on\-disk format at any time which will move the feature to the
  79 \fBactive\fR state. Some features may support returning to the \fBenabled\fR
  80 state after becoming \fBactive\fR. See feature\-specific documentation for
  81 details.
  82 .RE
  84 .sp
  85 .ne 2
  86 .na
  87 \fBdisabled\fR
  88 .ad
  89 .RS 12n
  90 This feature's on\-disk format changes have not been made and will not be made
  91 unless an administrator moves the feature to the \fBenabled\fR state. Features
  92 cannot be disabled once they have been enabled.
  93 .RE
  95 .sp
  96 .LP
  97 The state of supported features is exposed through pool properties of the form
  98 \fIfeature@short_name\fR.
  99 .SS "Read\-only compatibility"
 100 .sp
 101 .LP
 102 Some features may make on\-disk format changes that do not interfere with other
 103 software's ability to read from the pool. These features are referred to as
 104 "read\-only compatible". If all unsupported features on a pool are read\-only
 105 compatible, the pool can be imported in read\-only mode by setting the
 106 \fBreadonly\fR property during import (see \fBzpool\fR(1M) for details on
 107 importing pools).
 108 .SS "Unsupported features"
 109 .sp
 110 .LP
 111 For each unsupported feature enabled on an imported pool a pool property
 112 named \fIunsupported@feature_guid\fR will indicate why the import was allowed
 113 despite the unsupported feature. Possible values for this property are:
 115 .sp
 116 .ne 2
 117 .na
 118 \fB\fBinactive\fR\fR
 119 .ad
 120 .RS 12n
 121 The feature is in the \fBenabled\fR state and therefore the pool's on\-disk
 122 format is still compatible with software that does not support this feature.
 123 .RE
 125 .sp
 126 .ne 2
 127 .na
 128 \fB\fBreadonly\fR\fR
 129 .ad
 130 .RS 12n
 131 The feature is read\-only compatible and the pool has been imported in
 132 read\-only mode.
 133 .RE
 135 .SS "Feature dependencies"
 136 .sp
 137 .LP
 138 Some features depend on other features being enabled in order to function
 139 properly. Enabling a feature will automatically enable any features it
 140 depends on.
 142 .sp
 143 .LP
 144 The following features are supported on this system:
 145 .sp
 146 .ne 2
 147 .na
 148 \fB\fBasync_destroy\fR\fR
 149 .ad
 150 .RS 4n
 151 .TS
 152 l l .
 153 GUID    com.delphix:async_destroy
 155 DEPENDENCIES    none
 156 .TE
 158 Destroying a file system requires traversing all of its data in order to
 159 return its used space to the pool. Without \fBasync_destroy\fR the file system
 160 is not fully removed until all space has been reclaimed. If the destroy
 161 operation is interrupted by a reboot or power outage the next attempt to open
 162 the pool will need to complete the destroy operation synchronously.
 164 When \fBasync_destroy\fR is enabled the file system's data will be reclaimed
 165 by a background process, allowing the destroy operation to complete without
 166 traversing the entire file system. The background process is able to resume
 167 interrupted destroys after the pool has been opened, eliminating the need
 168 to finish interrupted destroys as part of the open operation. The amount
 169 of space remaining to be reclaimed by the background process is available
 170 through the \fBfreeing\fR property.
 172 This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero.
 173 .RE
 175 .sp
 176 .ne 2
 177 .na
 178 \fB\fBempty_bpobj\fR\fR
 179 .ad
 180 .RS 4n
 181 .TS
 182 l l .
 183 GUID    com.delphix:empty_bpobj
 185 DEPENDENCIES    none
 186 .TE
 188 This feature increases the performance of creating and using a large
 189 number of snapshots of a single filesystem or volume, and also reduces
 190 the disk space required.
 192 When there are many snapshots, each snapshot uses many Block Pointer
 193 Objects (bpobj's) to track blocks associated with that snapshot.
 194 However, in common use cases, most of these bpobj's are empty.  This
 195 feature allows us to create each bpobj on-demand, thus eliminating the
 196 empty bpobjs.
 198 This feature is \fBactive\fR while there are any filesystems, volumes,
 199 or snapshots which were created after enabling this feature.
 200 .RE
 202 .sp
 203 .ne 2
 204 .na
 205 \fB\fBlz4_compress\fR\fR
 206 .ad
 207 .RS 4n
 208 .TS
 209 l l .
 210 GUID    org.illumos:lz4_compress
 212 DEPENDENCIES    none
 213 .TE
 215 \fBlz4\fR is a high-performance real-time compression algorithm that
 216 features significantly faster compression and decompression as well as a
 217 higher compression ratio than the older \fBlzjb\fR compression.
 218 Typically, \fBlz4\fR compression is approximately 50% faster on
 219 compressible data and 200% faster on incompressible data than
 220 \fBlzjb\fR. It is also approximately 80% faster on decompression, while
 221 giving approximately 10% better compression ratio.
 223 When the \fBlz4_compress\fR feature is set to \fBenabled\fR, the
 224 administrator can turn on \fBlz4\fR compression on any dataset on the
 225 pool using the \fBzfs\fR(1M) command. Please note that doing so will
 226 immediately activate the \fBlz4_compress\fR feature on the underlying
 227 pool (even before any data is written). Since this feature is not
 228 read-only compatible, this operation will render the pool unimportable
 229 on systems without support for the \fBlz4_compress\fR feature. At the
 230 moment, this operation cannot be reversed. Booting off of
 231 \fBlz4\fR-compressed root pools is supported.
 232 .RE
 234 .sp
 235 .ne 2
 236 .na
 237 \fB\fBspacemap_histogram\fR\fR
 238 .ad
 239 .RS 4n
 240 .TS
 241 l l .
 242 GUID    com.delphix:spacemap_histogram
 244 DEPENDENCIES    none
 245 .TE
 247 This features allows ZFS to maintain more information about how free space
 248 is organized within the pool. If this feature is \fBenabled\fR, ZFS will
 249 set this feature to \fBactive\fR when a new space map object is created or
 250 an existing space map is upgraded to the new format. Once the feature is
 251 \fBactive\fR, it will remain in that state until the pool is destroyed.
 252 .RE
 254 .sp
 255 .ne 2
 256 .na
 257 \fB\fBmulti_vdev_crash_dump\fR\fR
 258 .ad
 259 .RS 4n
 260 .TS
 261 l l .
 262 GUID    com.joyent:multi_vdev_crash_dump
 264 DEPENDENCIES    none
 265 .TE
 267 This feature allows a dump device to be configured with a pool comprised
 268 of multiple vdevs.  Those vdevs may be arranged in any mirrored or raidz
 269 configuration.
 271 When the \fBmulti_vdev_crash_dump\fR feature is set to \fBenabled\fR,
 272 the administrator can use the \fBdumpadm\fR(1M) command to configure a
 273 dump device on a pool comprised of multiple vdevs.
 275 .SH "SEE ALSO"
 276 \fBzpool\fR(1M)