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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. Also, all newly written metadata
247 will be compressed with \fBlz4\fR algorithm. Since this feature is not
248 read-only compatible, this operation will render the pool unimportable
249 on systems without support for the \fBlz4_compress\fR feature. Booting
250 off of \fBlz4\fR-compressed root pools is supported.
251
252 This feature becomes \fBactive\fR as soon as it is enabled and will
253 never return to being \fBenabled\fB.
254 .RE
255
256 .sp
257 .ne 2
258 .na
259 \fB\fBspacemap_histogram\fR\fR
260 .ad
261 .RS 4n
262 .TS
263 l l .
264 GUID com.delphix:spacemap_histogram
265 READ\-ONLY COMPATIBLE yes
266 DEPENDENCIES none
267 .TE
268
269 This features allows ZFS to maintain more information about how free space
270 is organized within the pool. If this feature is \fBenabled\fR, ZFS will
271 set this feature to \fBactive\fR when a new space map object is created or
272 an existing space map is upgraded to the new format. Once the feature is
273 \fBactive\fR, it will remain in that state until the pool is destroyed.
274 .RE
275
276 .sp
277 .ne 2
278 .na
279 \fB\fBmulti_vdev_crash_dump\fR\fR
280 .ad
281 .RS 4n
282 .TS
283 l l .
284 GUID com.joyent:multi_vdev_crash_dump
285 READ\-ONLY COMPATIBLE no
286 DEPENDENCIES none
287 .TE
288
289 This feature allows a dump device to be configured with a pool comprised
290 of multiple vdevs. Those vdevs may be arranged in any mirrored or raidz
291 configuration.
292
293 When the \fBmulti_vdev_crash_dump\fR feature is set to \fBenabled\fR,
294 the administrator can use the \fBdumpadm\fR(1M) command to configure a
295 dump device on a pool comprised of multiple vdevs.
296 .RE
297
298 .sp
299 .ne 2
300 .na
301 \fB\fBextensible_dataset\fR\fR
302 .ad
303 .RS 4n
304 .TS
305 l l .
306 GUID com.delphix:extensible_dataset
307 READ\-ONLY COMPATIBLE no
308 DEPENDENCIES none
309 .TE
310
311 This feature allows more flexible use of internal ZFS data structures,
312 and exists for other features to depend on.
313
314 This feature will be \fBactive\fR when the first dependent feature uses it,
315 and will be returned to the \fBenabled\fR state when all datasets that use
316 this feature are destroyed.
317
318 .RE
319
320 .sp
321 .ne 2
322 .na
323 \fB\fBbookmarks\fR\fR
324 .ad
325 .RS 4n
326 .TS
327 l l .
328 GUID com.delphix:bookmarks
329 READ\-ONLY COMPATIBLE yes
330 DEPENDENCIES extensible_dataset
331 .TE
332
333 This feature enables use of the \fBzfs bookmark\fR subcommand.
334
335 This feature is \fBactive\fR while any bookmarks exist in the pool.
336 All bookmarks in the pool can be listed by running
337 \fBzfs list -t bookmark -r \fIpoolname\fR\fR.
338
339 .RE
340
341 .sp
342 .ne 2
343 .na
344 \fB\fBenabled_txg\fR\fR
345 .ad
346 .RS 4n
347 .TS
348 l l .
349 GUID com.delphix:enabled_txg
350 READ\-ONLY COMPATIBLE yes
351 DEPENDENCIES none
352 .TE
353
354 Once this feature is enabled ZFS records the transaction group number
355 in which new features are enabled. This has no user-visible impact,
356 but other features may depend on this feature.
357
358 This feature becomes \fBactive\fR as soon as it is enabled and will
359 never return to being \fBenabled\fB.
360
361 .RE
362
363 .sp
364 .ne 2
365 .na
366 \fB\fBhole_birth\fR\fR
367 .ad
368 .RS 4n
369 .TS
370 l l .
371 GUID com.delphix:hole_birth
372 READ\-ONLY COMPATIBLE no
373 DEPENDENCIES enabled_txg
374 .TE
375
376 This feature improves performance of incremental sends ("zfs send -i")
377 and receives for objects with many holes. The most common case of
378 hole-filled objects is zvols.
379
380 An incremental send stream from snapshot \fBA\fR to snapshot \fBB\fR
381 contains information about every block that changed between \fBA\fR and
382 \fBB\fR. Blocks which did not change between those snapshots can be
383 identified and omitted from the stream using a piece of metadata called
384 the 'block birth time', but birth times are not recorded for holes (blocks
385 filled only with zeroes). Since holes created after \fBA\fR cannot be
386 distinguished from holes created before \fBA\fR, information about every
387 hole in the entire filesystem or zvol is included in the send stream.
388
389 For workloads where holes are rare this is not a problem. However, when
390 incrementally replicating filesystems or zvols with many holes (for
391 example a zvol formatted with another filesystem) a lot of time will
392 be spent sending and receiving unnecessary information about holes that
393 already exist on the receiving side.
394
395 Once the \fBhole_birth\fR feature has been enabled the block birth times
396 of all new holes will be recorded. Incremental sends between snapshots
397 created after this feature is enabled will use this new metadata to avoid
398 sending information about holes that already exist on the receiving side.
399
400 This feature becomes \fBactive\fR as soon as it is enabled and will
401 never return to being \fBenabled\fB.
402
403 .RE
404
405
406 .SH "SEE ALSO"
407 \fBzpool\fR(1M)