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  15 .TH ZPOOL-FEATURES 5 "Mar 16, 2012"
  16 .SH NAME
  17 zpool\-features \- ZFS pool feature descriptions
  19 .sp
  20 .LP
  21 ZFS pool on\-disk format versions are specified via "features" which replace
  22 the old on\-disk format numbers (the last supported on\-disk format number is
  23 28). To enable a feature on a pool use the \fBupgrade\fR subcommand of the
  24 \fBzpool\fR(1M) command, or set the \fBfeature@\fR\fIfeature_name\fR property
  25 to \fBenabled\fR.
  26 .sp
  27 .LP
  28 The pool format does not affect file system version compatibility or the ability
  29 to send file systems between pools.
  30 .sp
  31 .LP
  32 Since most features can be enabled independently of each other the on\-disk
  33 format of the pool is specified by the set of all features marked as
  34 \fBactive\fR on the pool. If the pool was created by another software version
  35 this set may include unsupported features.
  36 .SS "Identifying features"
  37 .sp
  38 .LP
  39 Every feature has a guid of the form \fIcom.example:feature_name\fR. The reverse
  40 DNS name ensures that the feature's guid is unique across all ZFS
  41 implementations. When unsupported features are encountered on a pool they will
  42 be identified by their guids. Refer to the documentation for the ZFS
  43 implementation that created the pool for information about those features.
  44 .sp
  45 .LP
  46 Each supported feature also has a short name. By convention a feature's short
  47 name is the portion of its guid which follows the ':' (e.g.
  48 \fIcom.example:feature_name\fR would have the short name \fIfeature_name\fR),
  49 however a feature's short name may differ across ZFS implementations if
  50 following the convention would result in name conflicts.
  51 .SS "Feature states"
  52 .sp
  53 .LP
  54 Features can be in one of three states:
  55 .sp
  56 .ne 2
  57 .na
  58 \fB\fBactive\fR\fR
  59 .ad
  60 .RS 12n
  61 This feature's on\-disk format changes are in effect on the pool. Support for
  62 this feature is required to import the pool in read\-write mode. If this
  63 feature is not read-only compatible, support is also required to import the pool
  64 in read\-only mode (see "Read\-only compatibility").
  65 .RE
  67 .sp
  68 .ne 2
  69 .na
  70 \fB\fBenabled\fR\fR
  71 .ad
  72 .RS 12n
  73 An administrator has marked this feature as enabled on the pool, but the
  74 feature's on\-disk format changes have not been made yet. The pool can still be
  75 imported by software that does not support this feature, but changes may be made
  76 to the on\-disk format at any time which will move the feature to the
  77 \fBactive\fR state. Some features may support returning to the \fBenabled\fR
  78 state after becoming \fBactive\fR. See feature\-specific documentation for
  79 details.
  80 .RE
  82 .sp
  83 .ne 2
  84 .na
  85 \fBdisabled\fR
  86 .ad
  87 .RS 12n
  88 This feature's on\-disk format changes have not been made and will not be made
  89 unless an administrator moves the feature to the \fBenabled\fR state. Features
  90 cannot be disabled once they have been enabled.
  91 .RE
  93 .sp
  94 .LP
  95 The state of supported features is exposed through pool properties of the form
  96 \fIfeature@short_name\fR.
  97 .SS "Read\-only compatibility"
  98 .sp
  99 .LP
 100 Some features may make on\-disk format changes that do not interfere with other
 101 software's ability to read from the pool. These features are referred to as
 102 "read\-only compatible". If all unsupported features on a pool are read\-only
 103 compatible, the pool can be imported in read\-only mode by setting the
 104 \fBreadonly\fR property during import (see \fBzpool\fR(1M) for details on
 105 importing pools).
 106 .SS "Unsupported features"
 107 .sp
 108 .LP
 109 For each unsupported feature enabled on an imported pool a pool property
 110 named \fIunsupported@feature_guid\fR will indicate why the import was allowed
 111 despite the unsupported feature. Possible values for this property are:
 113 .sp
 114 .ne 2
 115 .na
 116 \fB\fBinactive\fR\fR
 117 .ad
 118 .RS 12n
 119 The feature is in the \fBenabled\fR state and therefore the pool's on\-disk
 120 format is still compatible with software that does not support this feature.
 121 .RE
 123 .sp
 124 .ne 2
 125 .na
 126 \fB\fBreadonly\fR\fR
 127 .ad
 128 .RS 12n
 129 The feature is read\-only compatible and the pool has been imported in
 130 read\-only mode.
 131 .RE
 133 .SS "Feature dependencies"
 134 .sp
 135 .LP
 136 Some features depend on other features being enabled in order to function
 137 properly. Enabling a feature will automatically enable any features it
 138 depends on.
 140 .sp
 141 .LP
 142 The following features are supported on this system:
 143 .sp
 144 .ne 2
 145 .na
 146 \fB\fBasync_destroy\fR\fR
 147 .ad
 148 .RS 4n
 149 .TS
 150 l l .
 151 GUID    com.delphix:async_destroy
 153 DEPENDENCIES    none
 154 .TE
 156 Destroying a file system requires traversing all of its data in order to
 157 return its used space to the pool. Without \fBasync_destroy\fR the file system
 158 is not fully removed until all space has been reclaimed. If the destroy
 159 operation is interrupted by a reboot or power outage the next attempt to open
 160 the pool will need to complete the destroy operation synchronously.
 162 When \fBasync_destroy\fR is enabled the file system's data will be reclaimed
 163 by a background process, allowing the destroy operation to complete without
 164 traversing the entire file system. The background process is able to resume
 165 interrupted destroys after the pool has been opened, eliminating the need
 166 to finish interrupted destroys as part of the open operation. The amount
 167 of space remaining to be reclaimed by the background process is available
 168 through the \fBfreeing\fR property.
 170 This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero.
 171 .RE
 173 .sp
 174 .ne 2
 175 .na
 176 \fB\fBempty_bpobj\fR\fR
 177 .ad
 178 .RS 4n
 179 .TS
 180 l l .
 181 GUID    com.delphix:empty_bpobj
 183 DEPENDENCIES    none
 184 .TE
 186 This feature increases the performance of creating and using a large
 187 number of snapshots of a single filesystem or volume, and also reduces
 188 the disk space required.
 190 When there are many snapshots, each snapshot uses many Block Pointer
 191 Objects (bpobj's) to track blocks associated with that snapshot.
 192 However, in common use cases, most of these bpobj's are empty.  This
 193 feature allows us to create each bpobj on-demand, thus eliminating the
 194 empty bpobjs.
 196 This feature is \fBactive\fR while there are any filesystems, volumes,
 197 or snapshots which were created after enabling this feature.
 198 .RE
 200 .SH "SEE ALSO"
 201 \fBzpool\fR(1M)