man/man5/zpool-features.5

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  16 .TH ZPOOL-FEATURES 5 "Aug 27, 2013"
  17 .SH NAME
  18 zpool\-features \- ZFS pool feature descriptions
  19 .SH DESCRIPTION
  20 .sp
  21 .LP
  22 ZFS pool on\-disk format versions are specified via "features" which replace
  23 the old on\-disk format numbers (the last supported on\-disk format number is
  24 28). To enable a feature on a pool use the \fBupgrade\fR subcommand of the
  25 \fBzpool\fR(8) command, or set the \fBfeature@\fR\fIfeature_name\fR property
  26 to \fBenabled\fR.
  27 .sp
  28 .LP
  29 The pool format does not affect file system version compatibility or the ability
  30 to send file systems between pools.
  31 .sp
  32 .LP
  33 Since most features can be enabled independently of each other the on\-disk
  34 format of the pool is specified by the set of all features marked as
  35 \fBactive\fR on the pool. If the pool was created by another software version
  36 this set may include unsupported features.
  37 .SS "Identifying features"
  38 .sp
  39 .LP
  40 Every feature has a guid of the form \fIcom.example:feature_name\fR. The reverse
  41 DNS name ensures that the feature's guid is unique across all ZFS
  42 implementations. When unsupported features are encountered on a pool they will
  43 be identified by their guids. Refer to the documentation for the ZFS
  44 implementation that created the pool for information about those features.
  45 .sp
  46 .LP
  47 Each supported feature also has a short name. By convention a feature's short
  48 name is the portion of its guid which follows the ':' (e.g.
  49 \fIcom.example:feature_name\fR would have the short name \fIfeature_name\fR),
  50 however a feature's short name may differ across ZFS implementations if
  51 following the convention would result in name conflicts.
  52 .SS "Feature states"
  53 .sp
  54 .LP
  55 Features can be in one of three states:
  56 .sp
  57 .ne 2
  58 .na
  59 \fB\fBactive\fR\fR
  60 .ad
  61 .RS 12n
  62 This feature's on\-disk format changes are in effect on the pool. Support for
  63 this feature is required to import the pool in read\-write mode. If this
  64 feature is not read-only compatible, support is also required to import the pool
  65 in read\-only mode (see "Read\-only compatibility").
  66 .RE
  67
  68 .sp
  69 .ne 2
  70 .na
  71 \fB\fBenabled\fR\fR
  72 .ad
  73 .RS 12n
  74 An administrator has marked this feature as enabled on the pool, but the
  75 feature's on\-disk format changes have not been made yet. The pool can still be
  76 imported by software that does not support this feature, but changes may be made
  77 to the on\-disk format at any time which will move the feature to the
  78 \fBactive\fR state. Some features may support returning to the \fBenabled\fR
  79 state after becoming \fBactive\fR. See feature\-specific documentation for
  80 details.
  81 .RE
  82
  83 .sp
  84 .ne 2
  85 .na
  86 \fBdisabled\fR
  87 .ad
  88 .RS 12n
  89 This feature's on\-disk format changes have not been made and will not be made
  90 unless an administrator moves the feature to the \fBenabled\fR state. Features
  91 cannot be disabled once they have been enabled.
  92 .RE
  93
  94 .sp
  95 .LP
  96 The state of supported features is exposed through pool properties of the form
  97 \fIfeature@short_name\fR.
  98 .SS "Read\-only compatibility"
  99 .sp
 100 .LP
 101 Some features may make on\-disk format changes that do not interfere with other
 102 software's ability to read from the pool. These features are referred to as
 103 "read\-only compatible". If all unsupported features on a pool are read\-only
 104 compatible, the pool can be imported in read\-only mode by setting the
 105 \fBreadonly\fR property during import (see \fBzpool\fR(8) for details on
 106 importing pools).
 107 .SS "Unsupported features"
 108 .sp
 109 .LP
 110 For each unsupported feature enabled on an imported pool a pool property
 111 named \fIunsupported@feature_guid\fR will indicate why the import was allowed
 112 despite the unsupported feature. Possible values for this property are:
 113
 114 .sp
 115 .ne 2
 116 .na
 117 \fB\fBinactive\fR\fR
 118 .ad
 119 .RS 12n
 120 The feature is in the \fBenabled\fR state and therefore the pool's on\-disk
 121 format is still compatible with software that does not support this feature.
 122 .RE
 123
 124 .sp
 125 .ne 2
 126 .na
 127 \fB\fBreadonly\fR\fR
 128 .ad
 129 .RS 12n
 130 The feature is read\-only compatible and the pool has been imported in
 131 read\-only mode.
 132 .RE
 133
 134 .SS "Feature dependencies"
 135 .sp
 136 .LP
 137 Some features depend on other features being enabled in order to function
 138 properly. Enabling a feature will automatically enable any features it
 139 depends on.
 140 .SH FEATURES
 141 .sp
 142 .LP
 143 The following features are supported on this system:
 144 .sp
 145 .ne 2
 146 .na
 147 \fB\fBasync_destroy\fR\fR
 148 .ad
 149 .RS 4n
 150 .TS
 151 l l .
 152 GUID    com.delphix:async_destroy
 153 READ\-ONLY COMPATIBLE   yes
 154 DEPENDENCIES    none
 155 .TE
 156
 157 Destroying a file system requires traversing all of its data in order to
 158 return its used space to the pool. Without \fBasync_destroy\fR the file system
 159 is not fully removed until all space has been reclaimed. If the destroy
 160 operation is interrupted by a reboot or power outage the next attempt to open
 161 the pool will need to complete the destroy operation synchronously.
 162
 163 When \fBasync_destroy\fR is enabled the file system's data will be reclaimed
 164 by a background process, allowing the destroy operation to complete without
 165 traversing the entire file system. The background process is able to resume
 166 interrupted destroys after the pool has been opened, eliminating the need
 167 to finish interrupted destroys as part of the open operation. The amount
 168 of space remaining to be reclaimed by the background process is available
 169 through the \fBfreeing\fR property.
 170
 171 This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero.
 172 .RE
 173
 174 .sp
 175 .ne 2
 176 .na
 177 \fB\fBempty_bpobj\fR\fR
 178 .ad
 179 .RS 4n
 180 .TS
 181 l l .
 182 GUID    com.delphix:empty_bpobj
 183 READ\-ONLY COMPATIBLE   yes
 184 DEPENDENCIES    none
 185 .TE
 186
 187 This feature increases the performance of creating and using a large
 188 number of snapshots of a single filesystem or volume, and also reduces
 189 the disk space required.
 190
 191 When there are many snapshots, each snapshot uses many Block Pointer
 192 Objects (bpobj's) to track blocks associated with that snapshot.
 193 However, in common use cases, most of these bpobj's are empty.  This
 194 feature allows us to create each bpobj on-demand, thus eliminating the
 195 empty bpobjs.
 196
 197 This feature is \fBactive\fR while there are any filesystems, volumes,
 198 or snapshots which were created after enabling this feature.
 199 .RE
 200
 201 .sp
 202 .ne 2
 203 .na
 204 \fB\fBlz4_compress\fR\fR
 205 .ad
 206 .RS 4n
 207 .TS
 208 l l .
 209 GUID    org.illumos:lz4_compress
 210 READ\-ONLY COMPATIBLE   no
 211 DEPENDENCIES    none
 212 .TE
 213
 214 \fBlz4\fR is a high-performance real-time compression algorithm that
 215 features significantly faster compression and decompression as well as a
 216 higher compression ratio than the older \fBlzjb\fR compression.
 217 Typically, \fBlz4\fR compression is approximately 50% faster on
 218 compressible data and 200% faster on incompressible data than
 219 \fBlzjb\fR. It is also approximately 80% faster on decompression, while
 220 giving approximately 10% better compression ratio.
 221
 222 When the \fBlz4_compress\fR feature is set to \fBenabled\fR, the
 223 administrator can turn on \fBlz4\fR compression on any dataset on the
 224 pool using the \fBzfs\fR(8) command. Please note that doing so will
 225 immediately activate the \fBlz4_compress\fR feature on the underlying
 226 pool using the \fBzfs\fR(1M) command. Also, all newly written metadata
 227 will be compressed with \fBlz4\fR algorithm. 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. Booting
 230 off of \fBlz4\fR-compressed root pools is supported.
 231
 232 This feature becomes \fBactive\fR as soon as it is enabled and will
 233 never return to being \fBenabled\fB.
 234 .RE
 235
 236 .sp
 237 .ne 2
 238 .na
 239 \fB\fBspacemap_histogram\fR\fR
 240 .ad
 241 .RS 4n
 242 .TS
 243 l l .
 244 GUID    com.delphix:spacemap_histogram
 245 READ\-ONLY COMPATIBLE   yes
 246 DEPENDENCIES    none
 247 .TE
 248
 249 This features allows ZFS to maintain more information about how free space
 250 is organized within the pool. If this feature is \fBenabled\fR, ZFS will
 251 set this feature to \fBactive\fR when a new space map object is created or
 252 an existing space map is upgraded to the new format. Once the feature is
 253 \fBactive\fR, it will remain in that state until the pool is destroyed.
 254
 255 .RE
 256
 257 .sp
 258 .ne 2
 259 .na
 260 \fB\fBextensible_dataset\fR\fR
 261 .ad
 262 .RS 4n
 263 .TS
 264 l l .
 265 GUID    com.delphix:extensible_dataset
 266 READ\-ONLY COMPATIBLE   no
 267 DEPENDENCIES    none
 268 .TE
 269
 270 This feature allows more flexible use of internal ZFS data structures,
 271 and exists for other features to depend on.
 272
 273 This feature will be \fBactive\fR when the first dependent feature uses it,
 274 and will be returned to the \fBenabled\fR state when all datasets that use
 275 this feature are destroyed.
 276
 277 .RE
 278
 279 .sp
 280 .ne 2
 281 .na
 282 \fB\fBbookmarks\fR\fR
 283 .ad
 284 .RS 4n
 285 .TS
 286 l l .
 287 GUID    com.delphix:bookmarks
 288 READ\-ONLY COMPATIBLE   yes
 289 DEPENDENCIES    extensible_dataset
 290 .TE
 291
 292 This feature enables use of the \fBzfs bookmark\fR subcommand.
 293
 294 This feature is \fBactive\fR while any bookmarks exist in the pool.
 295 All bookmarks in the pool can be listed by running
 296 \fBzfs list -t bookmark -r \fIpoolname\fR\fR.
 297
 298 .RE
 299
 300 .sp
 301 .ne 2
 302 .na
 303 \fB\fBenabled_txg\fR\fR
 304 .ad
 305 .RS 4n
 306 .TS
 307 l l .
 308 GUID    com.delphix:enabled_txg
 309 READ\-ONLY COMPATIBLE   yes
 310 DEPENDENCIES    none
 311 .TE
 312
 313 Once this feature is enabled ZFS records the transaction group number
 314 in which new features are enabled. This has no user-visible impact,
 315 but other features may depend on this feature.
 316
 317 This feature becomes \fBactive\fR as soon as it is enabled and will
 318 never return to being \fBenabled\fB.
 319
 320 .RE
 321
 322 .sp
 323 .ne 2
 324 .na
 325 \fB\fBhole_birth\fR\fR
 326 .ad
 327 .RS 4n
 328 .TS
 329 l l .
 330 GUID    com.delphix:hole_birth
 331 READ\-ONLY COMPATIBLE   no
 332 DEPENDENCIES    enabled_txg
 333 .TE
 334
 335 This feature improves performance of incremental sends ("zfs send -i")
 336 and receives for objects with many holes. The most common case of
 337 hole-filled objects is zvols.
 338
 339 An incremental send stream from snapshot \fBA\fR to snapshot \fBB\fR
 340 contains information about every block that changed between \fBA\fR and
 341 \fBB\fR. Blocks which did not change between those snapshots can be
 342 identified and omitted from the stream using a piece of metadata called
 343 the 'block birth time', but birth times are not recorded for holes (blocks
 344 filled only with zeroes). Since holes created after \fBA\fR cannot be
 345 distinguished from holes created before \fBA\fR, information about every
 346 hole in the entire filesystem or zvol is included in the send stream.
 347
 348 For workloads where holes are rare this is not a problem. However, when
 349 incrementally replicating filesystems or zvols with many holes (for
 350 example a zvol formatted with another filesystem) a lot of time will
 351 be spent sending and receiving unnecessary information about holes that
 352 already exist on the receiving side.
 353
 354 Once the \fBhole_birth\fR feature has been enabled the block birth times
 355 of all new holes will be recorded. Incremental sends between snapshots
 356 created after this feature is enabled will use this new metadata to avoid
 357 sending information about holes that already exist on the receiving side.
 358
 359 This feature becomes \fBactive\fR as soon as it is enabled and will
 360 never return to being \fBenabled\fB.
 361
 362 .RE
 363
 364 .sp
 365 .ne 2
 366 .na
 367 \fB\fBembedded_data\fR\fR
 368 .ad
 369 .RS 4n
 370 .TS
 371 l l .
 372 GUID    com.delphix:embedded_data
 373 READ\-ONLY COMPATIBLE   no
 374 DEPENDENCIES    none
 375 .TE
 376
 377 This feature improves the performance and compression ratio of
 378 highly-compressible blocks.  Blocks whose contents can compress to 112 bytes
 379 or smaller can take advantage of this feature.
 380
 381 When this feature is enabled, the contents of highly-compressible blocks are
 382 stored in the block "pointer" itself (a misnomer in this case, as it contains
 383 the compresseed data, rather than a pointer to its location on disk).  Thus
 384 the space of the block (one sector, typically 512 bytes or 4KB) is saved,
 385 and no additional i/o is needed to read and write the data block.
 386
 387 This feature becomes \fBactive\fR as soon as it is enabled and will
 388 never return to being \fBenabled\fR.
 389
 390 .RE
 391
 392 .SH "SEE ALSO"
 393 \fBzpool\fR(8)