1 '\" te
   2 .\" Copyright (c) 2013 by Delphix. All rights reserved.
   3 .\" Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
   4 .\" Copyright (c) 2014, Joyent, Inc. All rights reserved.
   5 .\" The contents of this file are subject to the terms of the Common Development
   6 .\" and Distribution License (the "License").  You may not use this file except
   7 .\" in compliance with the License. You can obtain a copy of the license at
   8 .\" usr/src/OPENSOLARIS.LICENSE or http://www.opensolaris.org/os/licensing.
   9 .\"
  10 .\" See the License for the specific language governing permissions and
  11 .\" limitations under the License. When distributing Covered Code, include this
  12 .\" CDDL HEADER in each file and include the License file at
  13 .\" usr/src/OPENSOLARIS.LICENSE.  If applicable, add the following below this
  14 .\" CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your
  15 .\" own identifying information:
  16 .\" Portions Copyright [yyyy] [name of copyright owner]
  17 .TH ZPOOL-FEATURES 5 "Aug 27, 2013"
  18 .SH NAME
  19 zpool\-features \- ZFS pool feature descriptions
  20 .SH DESCRIPTION
  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
  68 
  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
  83 
  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
  94 
  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:
 114 
 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
 124 
 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
 134 
 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.
 141 .SH FEATURES
 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
 154 READ\-ONLY COMPATIBLE   yes
 155 DEPENDENCIES    none
 156 .TE
 157 
 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.
 163 
 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.
 171 
 172 This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero.
 173 .RE
 174 
 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
 184 READ\-ONLY COMPATIBLE   yes
 185 DEPENDENCIES    none
 186 .TE
 187 
 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.
 191 
 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.
 197 
 198 This feature is \fBactive\fR while there are any filesystems, volumes,
 199 or snapshots which were created after enabling this feature.
 200 .RE
 201 
 202 .sp
 203 .ne 2
 204 .na
 205 \fB\fBfilesystem_limits\fR\fR
 206 .ad
 207 .RS 4n
 208 .TS
 209 l l .
 210 GUID    com.joyent:filesystem_limits
 211 READ\-ONLY COMPATIBLE   yes
 212 DEPENDENCIES    extensible_dataset
 213 .TE
 214 
 215 This feature enables filesystem and snapshot limits. These limits can be used
 216 to control how many filesystems and/or snapshots can be created at the point in
 217 the tree on which the limits are set.
 218 
 219 This feature is \fBactive\fR once either of the limit properties has been
 220 set on a dataset. Once activated the feature is never deactivated.
 221 .RE
 222 
 223 .sp
 224 .ne 2
 225 .na
 226 \fB\fBlz4_compress\fR\fR
 227 .ad
 228 .RS 4n
 229 .TS
 230 l l .
 231 GUID    org.illumos:lz4_compress
 232 READ\-ONLY COMPATIBLE   no
 233 DEPENDENCIES    none
 234 .TE
 235 
 236 \fBlz4\fR is a high-performance real-time compression algorithm that
 237 features significantly faster compression and decompression as well as a
 238 higher compression ratio than the older \fBlzjb\fR compression.
 239 Typically, \fBlz4\fR compression is approximately 50% faster on
 240 compressible data and 200% faster on incompressible data than
 241 \fBlzjb\fR. It is also approximately 80% faster on decompression, while
 242 giving approximately 10% better compression ratio.
 243 
 244 When the \fBlz4_compress\fR feature is set to \fBenabled\fR, the
 245 administrator can turn on \fBlz4\fR compression on any dataset on the
 246 pool using the \fBzfs\fR(1M) command. Please note that doing so will
 247 immediately activate the \fBlz4_compress\fR feature on the underlying
 248 pool (even before any data is written), and the feature will not be
 249 deactivated. Since this feature is not read-only compatible, this
 250 operation will render the pool unimportable on systems without support
 251 for the \fBlz4_compress\fR feature. Booting off of \fBlz4\fR-compressed
 252 root pools is supported.
 253 .RE
 254 
 255 .sp
 256 .ne 2
 257 .na
 258 \fB\fBspacemap_histogram\fR\fR
 259 .ad
 260 .RS 4n
 261 .TS
 262 l l .
 263 GUID    com.delphix:spacemap_histogram
 264 READ\-ONLY COMPATIBLE   yes
 265 DEPENDENCIES    none
 266 .TE
 267 
 268 This features allows ZFS to maintain more information about how free space
 269 is organized within the pool. If this feature is \fBenabled\fR, ZFS will
 270 set this feature to \fBactive\fR when a new space map object is created or
 271 an existing space map is upgraded to the new format. Once the feature is
 272 \fBactive\fR, it will remain in that state until the pool is destroyed.
 273 .RE
 274 
 275 .sp
 276 .ne 2
 277 .na
 278 \fB\fBmulti_vdev_crash_dump\fR\fR
 279 .ad
 280 .RS 4n
 281 .TS
 282 l l .
 283 GUID    com.joyent:multi_vdev_crash_dump
 284 READ\-ONLY COMPATIBLE   no
 285 DEPENDENCIES    none
 286 .TE
 287 
 288 This feature allows a dump device to be configured with a pool comprised
 289 of multiple vdevs.  Those vdevs may be arranged in any mirrored or raidz
 290 configuration.
 291 
 292 When the \fBmulti_vdev_crash_dump\fR feature is set to \fBenabled\fR,
 293 the administrator can use the \fBdumpadm\fR(1M) command to configure a
 294 dump device on a pool comprised of multiple vdevs.
 295 .RE
 296 
 297 .sp
 298 .ne 2
 299 .na
 300 \fB\fBextensible_dataset\fR\fR
 301 .ad
 302 .RS 4n
 303 .TS
 304 l l .
 305 GUID    com.delphix:extensible_dataset
 306 READ\-ONLY COMPATIBLE   no
 307 DEPENDENCIES    none
 308 .TE
 309 
 310 This feature allows more flexible use of internal ZFS data structures,
 311 and exists for other features to depend on.
 312 
 313 This feature will be \fBactive\fR when the first dependent feature uses it,
 314 and will be returned to the \fBenabled\fR state when all datasets that use
 315 this feature are destroyed.
 316 
 317 .RE
 318 
 319 .sp
 320 .ne 2
 321 .na
 322 \fB\fBbookmarks\fR\fR
 323 .ad
 324 .RS 4n
 325 .TS
 326 l l .
 327 GUID    com.delphix:bookmarks
 328 READ\-ONLY COMPATIBLE   yes
 329 DEPENDENCIES    extensible_dataset
 330 .TE
 331 
 332 This feature enables use of the \fBzfs bookmark\fR subcommand.
 333 
 334 This feature is \fBactive\fR while any bookmarks exist in the pool.
 335 All bookmarks in the pool can be listed by running
 336 \fBzfs list -t bookmark -r \fIpoolname\fR\fR.
 337 
 338 .RE
 339 
 340 .sp
 341 .ne 2
 342 .na
 343 \fB\fBenabled_txg\fR\fR
 344 .ad
 345 .RS 4n
 346 .TS
 347 l l .
 348 GUID    com.delphix:enabled_txg
 349 READ\-ONLY COMPATIBLE   yes
 350 DEPENDENCIES    none
 351 .TE
 352 
 353 Once this feature is enabled ZFS records the transaction group number
 354 in which new features are enabled. This has no user-visible impact,
 355 but other features may depend on this feature.
 356 
 357 This feature becomes \fBactive\fR as soon as it is enabled and will
 358 never return to being \fBenabled\fB.
 359 
 360 .RE
 361 
 362 .sp
 363 .ne 2
 364 .na
 365 \fB\fBhole_birth\fR\fR
 366 .ad
 367 .RS 4n
 368 .TS
 369 l l .
 370 GUID    com.delphix:hole_birth
 371 READ\-ONLY COMPATIBLE   no
 372 DEPENDENCIES    enabled_txg
 373 .TE
 374 
 375 This feature improves performance of incremental sends ("zfs send -i")
 376 and receives for objects with many holes. The most common case of
 377 hole-filled objects is zvols.
 378 
 379 An incremental send stream from snapshot \fBA\fR to snapshot \fBB\fR
 380 contains information about every block that changed between \fBA\fR and
 381 \fBB\fR. Blocks which did not change between those snapshots can be
 382 identified and omitted from the stream using a piece of metadata called
 383 the 'block birth time', but birth times are not recorded for holes (blocks
 384 filled only with zeroes). Since holes created after \fBA\fR cannot be
 385 distinguished from holes created before \fBA\fR, information about every
 386 hole in the entire filesystem or zvol is included in the send stream.
 387 
 388 For workloads where holes are rare this is not a problem. However, when
 389 incrementally replicating filesystems or zvols with many holes (for
 390 example a zvol formatted with another filesystem) a lot of time will
 391 be spent sending and receiving unnecessary information about holes that
 392 already exist on the receiving side.
 393 
 394 Once the \fBhole_birth\fR feature has been enabled the block birth times
 395 of all new holes will be recorded. Incremental sends between snapshots
 396 created after this feature is enabled will use this new metadata to avoid
 397 sending information about holes that already exist on the receiving side.
 398 
 399 This feature becomes \fBactive\fR as soon as it is enabled and will
 400 never return to being \fBenabled\fB.
 401 
 402 .RE
 403 
 404 
 405 .SH "SEE ALSO"
 406 \fBzpool\fR(1M)