4171 clean up spa_feature_*() interfaces
4172 implement extensible_dataset feature for use by other zpool features
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

          --- old/usr/src/man/man5/zpool-features.5
          +++ new/usr/src/man/man5/zpool-features.5

   1    1  '\" te
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  17   17  .TH ZPOOL-FEATURES 5 "Aug 27, 2013"
  18   18  .SH NAME
  19   19  zpool\-features \- ZFS pool feature descriptions
  20   20  .SH DESCRIPTION
  21   21  .sp
  22   22  .LP
  23   23  ZFS pool on\-disk format versions are specified via "features" which replace
  24   24  the old on\-disk format numbers (the last supported on\-disk format number is
  25   25  28). To enable a feature on a pool use the \fBupgrade\fR subcommand of the
  26   26  \fBzpool\fR(1M) command, or set the \fBfeature@\fR\fIfeature_name\fR property
  27   27  to \fBenabled\fR.
  28   28  .sp
  29   29  .LP
  30   30  The pool format does not affect file system version compatibility or the ability
  31   31  to send file systems between pools.
  32   32  .sp
  33   33  .LP
  34   34  Since most features can be enabled independently of each other the on\-disk
  35   35  format of the pool is specified by the set of all features marked as
  36   36  \fBactive\fR on the pool. If the pool was created by another software version
  37   37  this set may include unsupported features.
  38   38  .SS "Identifying features"
  39   39  .sp
  40   40  .LP
  41   41  Every feature has a guid of the form \fIcom.example:feature_name\fR. The reverse
  42   42  DNS name ensures that the feature's guid is unique across all ZFS
  43   43  implementations. When unsupported features are encountered on a pool they will
  44   44  be identified by their guids. Refer to the documentation for the ZFS
  45   45  implementation that created the pool for information about those features.
  46   46  .sp
  47   47  .LP
  48   48  Each supported feature also has a short name. By convention a feature's short
  49   49  name is the portion of its guid which follows the ':' (e.g.
  50   50  \fIcom.example:feature_name\fR would have the short name \fIfeature_name\fR),
  51   51  however a feature's short name may differ across ZFS implementations if
  52   52  following the convention would result in name conflicts.
  53   53  .SS "Feature states"
  54   54  .sp
  55   55  .LP
  56   56  Features can be in one of three states:
  57   57  .sp
  58   58  .ne 2
  59   59  .na
  60   60  \fB\fBactive\fR\fR
  61   61  .ad
  62   62  .RS 12n
  63   63  This feature's on\-disk format changes are in effect on the pool. Support for
  64   64  this feature is required to import the pool in read\-write mode. If this
  65   65  feature is not read-only compatible, support is also required to import the pool
  66   66  in read\-only mode (see "Read\-only compatibility").
  67   67  .RE
  68   68  
  69   69  .sp
  70   70  .ne 2
  71   71  .na
  72   72  \fB\fBenabled\fR\fR
  73   73  .ad
  74   74  .RS 12n
  75   75  An administrator has marked this feature as enabled on the pool, but the
  76   76  feature's on\-disk format changes have not been made yet. The pool can still be
  77   77  imported by software that does not support this feature, but changes may be made
  78   78  to the on\-disk format at any time which will move the feature to the
  79   79  \fBactive\fR state. Some features may support returning to the \fBenabled\fR
  80   80  state after becoming \fBactive\fR. See feature\-specific documentation for
  81   81  details.
  82   82  .RE
  83   83  
  84   84  .sp
  85   85  .ne 2
  86   86  .na
  87   87  \fBdisabled\fR
  88   88  .ad
  89   89  .RS 12n
  90   90  This feature's on\-disk format changes have not been made and will not be made
  91   91  unless an administrator moves the feature to the \fBenabled\fR state. Features
  92   92  cannot be disabled once they have been enabled.
  93   93  .RE
  94   94  
  95   95  .sp
  96   96  .LP
  97   97  The state of supported features is exposed through pool properties of the form
  98   98  \fIfeature@short_name\fR.
  99   99  .SS "Read\-only compatibility"
 100  100  .sp
 101  101  .LP
 102  102  Some features may make on\-disk format changes that do not interfere with other
 103  103  software's ability to read from the pool. These features are referred to as
 104  104  "read\-only compatible". If all unsupported features on a pool are read\-only
 105  105  compatible, the pool can be imported in read\-only mode by setting the
 106  106  \fBreadonly\fR property during import (see \fBzpool\fR(1M) for details on
 107  107  importing pools).
 108  108  .SS "Unsupported features"
 109  109  .sp
 110  110  .LP
 111  111  For each unsupported feature enabled on an imported pool a pool property
 112  112  named \fIunsupported@feature_guid\fR will indicate why the import was allowed
 113  113  despite the unsupported feature. Possible values for this property are:
 114  114  
 115  115  .sp
 116  116  .ne 2
 117  117  .na
 118  118  \fB\fBinactive\fR\fR
 119  119  .ad
 120  120  .RS 12n
 121  121  The feature is in the \fBenabled\fR state and therefore the pool's on\-disk
 122  122  format is still compatible with software that does not support this feature.
 123  123  .RE
 124  124  
 125  125  .sp
 126  126  .ne 2
 127  127  .na
 128  128  \fB\fBreadonly\fR\fR
 129  129  .ad
 130  130  .RS 12n
 131  131  The feature is read\-only compatible and the pool has been imported in
 132  132  read\-only mode.
 133  133  .RE
 134  134  
 135  135  .SS "Feature dependencies"
 136  136  .sp
 137  137  .LP
 138  138  Some features depend on other features being enabled in order to function
 139  139  properly. Enabling a feature will automatically enable any features it
 140  140  depends on.
 141  141  .SH FEATURES
 142  142  .sp
 143  143  .LP
 144  144  The following features are supported on this system:
 145  145  .sp
 146  146  .ne 2
 147  147  .na
 148  148  \fB\fBasync_destroy\fR\fR
 149  149  .ad
 150  150  .RS 4n
 151  151  .TS
 152  152  l l .
 153  153  GUID    com.delphix:async_destroy
 154  154  READ\-ONLY COMPATIBLE   yes
 155  155  DEPENDENCIES    none
 156  156  .TE
 157  157  
 158  158  Destroying a file system requires traversing all of its data in order to
 159  159  return its used space to the pool. Without \fBasync_destroy\fR the file system
 160  160  is not fully removed until all space has been reclaimed. If the destroy
 161  161  operation is interrupted by a reboot or power outage the next attempt to open
 162  162  the pool will need to complete the destroy operation synchronously.
 163  163  
 164  164  When \fBasync_destroy\fR is enabled the file system's data will be reclaimed
 165  165  by a background process, allowing the destroy operation to complete without
 166  166  traversing the entire file system. The background process is able to resume
 167  167  interrupted destroys after the pool has been opened, eliminating the need
 168  168  to finish interrupted destroys as part of the open operation. The amount
 169  169  of space remaining to be reclaimed by the background process is available
 170  170  through the \fBfreeing\fR property.
 171  171  
 172  172  This feature is only \fBactive\fR while \fBfreeing\fR is non\-zero.
 173  173  .RE
 174  174  
 175  175  .sp
 176  176  .ne 2
 177  177  .na
 178  178  \fB\fBempty_bpobj\fR\fR
 179  179  .ad
 180  180  .RS 4n
 181  181  .TS
 182  182  l l .
 183  183  GUID    com.delphix:empty_bpobj
 184  184  READ\-ONLY COMPATIBLE   yes
 185  185  DEPENDENCIES    none
 186  186  .TE
 187  187  
 188  188  This feature increases the performance of creating and using a large
 189  189  number of snapshots of a single filesystem or volume, and also reduces
 190  190  the disk space required.
 191  191  
 192  192  When there are many snapshots, each snapshot uses many Block Pointer
 193  193  Objects (bpobj's) to track blocks associated with that snapshot.
 194  194  However, in common use cases, most of these bpobj's are empty.  This
 195  195  feature allows us to create each bpobj on-demand, thus eliminating the
 196  196  empty bpobjs.
 197  197  
 198  198  This feature is \fBactive\fR while there are any filesystems, volumes,
 199  199  or snapshots which were created after enabling this feature.
 200  200  .RE
 201  201  
 202  202  .sp
 203  203  .ne 2
 204  204  .na
 205  205  \fB\fBlz4_compress\fR\fR
 206  206  .ad
 207  207  .RS 4n
 208  208  .TS
 209  209  l l .
 210  210  GUID    org.illumos:lz4_compress
 211  211  READ\-ONLY COMPATIBLE   no
 212  212  DEPENDENCIES    none
 213  213  .TE
 214  214  
 215  215  \fBlz4\fR is a high-performance real-time compression algorithm that
 216  216  features significantly faster compression and decompression as well as a
 217  217  higher compression ratio than the older \fBlzjb\fR compression.
 218  218  Typically, \fBlz4\fR compression is approximately 50% faster on
 219  219  compressible data and 200% faster on incompressible data than
 220  220  \fBlzjb\fR. It is also approximately 80% faster on decompression, while
 221  221  giving approximately 10% better compression ratio.
 222  222  
 223  223  When the \fBlz4_compress\fR feature is set to \fBenabled\fR, the
 224  224  administrator can turn on \fBlz4\fR compression on any dataset on the
 225  225  pool using the \fBzfs\fR(1M) command. Please note that doing so will
 226  226  immediately activate the \fBlz4_compress\fR feature on the underlying
 227  227  pool (even before any data is written). Since this feature is not
 228  228  read-only compatible, this operation will render the pool unimportable
 229  229  on systems without support for the \fBlz4_compress\fR feature. At the
 230  230  moment, this operation cannot be reversed. Booting off of
 231  231  \fBlz4\fR-compressed root pools is supported.
 232  232  .RE
 233  233  
 234  234  .sp
 235  235  .ne 2
 236  236  .na
 237  237  \fB\fBspacemap_histogram\fR\fR
 238  238  .ad
 239  239  .RS 4n
 240  240  .TS
 241  241  l l .
 242  242  GUID    com.delphix:spacemap_histogram
 243  243  READ\-ONLY COMPATIBLE   yes
 244  244  DEPENDENCIES    none
 245  245  .TE
 246  246  
 247  247  This features allows ZFS to maintain more information about how free space
 248  248  is organized within the pool. If this feature is \fBenabled\fR, ZFS will
 249  249  set this feature to \fBactive\fR when a new space map object is created or
 250  250  an existing space map is upgraded to the new format. Once the feature is
 251  251  \fBactive\fR, it will remain in that state until the pool is destroyed.
 252  252  .RE
 253  253  
 254  254  .sp
 255  255  .ne 2
 256  256  .na
 257  257  \fB\fBmulti_vdev_crash_dump\fR\fR
 258  258  .ad
 259  259  .RS 4n
 260  260  .TS
 261  261  l l .
 262  262  GUID    com.joyent:multi_vdev_crash_dump
 263  263  READ\-ONLY COMPATIBLE   no

 264  264  DEPENDENCIES    none
 265  265  .TE
 266  266  
 267  267  This feature allows a dump device to be configured with a pool comprised
 268  268  of multiple vdevs.  Those vdevs may be arranged in any mirrored or raidz
 269  269  configuration.
 270  270  
 271  271  When the \fBmulti_vdev_crash_dump\fR feature is set to \fBenabled\fR,
 272  272  the administrator can use the \fBdumpadm\fR(1M) command to configure a
 273  273  dump device on a pool comprised of multiple vdevs.
      274 +.RE
 274  275  
      276 +.sp
      277 +.ne 2
      278 +.na
      279 +\fB\fBextensible_dataset\fR\fR
      280 +.ad
      281 +.RS 4n
      282 +.TS
      283 +l l .
      284 +GUID    com.delphix:extensible_dataset
      285 +READ\-ONLY COMPATIBLE   no
      286 +DEPENDENCIES    none
      287 +.TE
      288 +
      289 +This feature allows more flexible use of internal ZFS data structures,
      290 +and exists for other features to depend on.
      291 +
      292 +This feature will be \fBactive\fR when the first dependent feature uses it,
      293 +and will be returned to the \fBenabled\fR state when all datasets that use
      294 +this feature are destroyed.
      295 +
      296 +.RE
      297 +
 275  298  .SH "SEE ALSO"
 276  299  \fBzpool\fR(1M)
    

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