ceph/doc/rbd/rbd-live-migration.rst

   1 ======================
   2  Image Live-Migration
   3 ======================
   4
   5 .. index:: Ceph Block Device; live-migration
   6
   7 RBD images can be live-migrated between different pools within the same cluster
   8 or between different image formats and layouts. When started, the source image
   9 will be deep-copied to the destination image, pulling all snapshot history and
  10 optionally keeping any link to the source image's parent to help preserve
  11 sparseness.
  12
  13 This copy process can safely run in the background while the new target image is
  14 in-use. There is currently a requirement to temporarily stop using the source
  15 image before preparing a migration. This helps to ensure that the client using
  16 the image is updated to point to the new target image.
  17
  18 .. note::
  19    Image live-migration requires the Ceph Nautilus release or later. The krbd
  20    kernel module does not support live-migration at this time.
  21
  22
  23 .. ditaa:: +-------------+               +-------------+
  24            | {s} c999    |               | {s}         |
  25            |  Live       | Target refers |  Live       |
  26            |  migration  |<-------------*|  migration  |
  27            |  source     |  to Source    |  target     |
  28            |             |               |             |
  29            | (read only) |               | (writable)  |
  30            +-------------+               +-------------+
  31
  32                Source                        Target
  33
  34 The live-migration process is comprised of three steps:
  35
  36 #. **Prepare Migration:** The initial step creates the new target image and
  37    cross-links the source and target images. Similar to `layered images`_,
  38    attempts to read uninitialized extents within the target image will
  39    internally redirect the read to the source image, and writes to
  40    uninitialized extents within the target will internally deep-copy the
  41    overlapping source image block to the target image.
  42
  43
  44 #. **Execute Migration:** This is a background operation that deep-copies all
  45    initialized blocks from the source image to the target. This step can be
  46    run while clients are actively using the new target image.
  47
  48
  49 #. **Finish Migration:** Once the background migration process has completed,
  50    the migration can be committed or aborted. Committing the migration will
  51    remove the cross-links between the source and target images, and will
  52    remove the source image. Aborting the migration will remove the cross-links,
  53    and will remove the target image.
  54
  55 Prepare Migration
  56 =================
  57
  58 The live-migration process is initiated by running the `rbd migration prepare`
  59 command, providing the source and target images::
  60
  61     $ rbd migration prepare migration_source [migration_target]
  62
  63 The `rbd migration prepare` command accepts all the same layout optionals as the
  64 `rbd create` command, which allows changes to the immutable image on-disk
  65 layout. The `migration_target` can be skipped if the goal is only to change the
  66 on-disk layout, keeping the original image name.
  67
  68 All clients using the source image must be stopped prior to preparing a
  69 live-migration. The prepare step will fail if it finds any running clients with
  70 the image open in read/write mode. Once the prepare step is complete, the
  71 clients can be restarted using the new target image name. Attempting to restart
  72 the clients using the source image name will result in failure.
  73
  74 The `rbd status` command will show the current state of the live-migration::
  75
  76     $ rbd status migration_target
  77     Watchers: none
  78     Migration:
  79                 source: rbd/migration_source (5e2cba2f62e)
  80                 destination: rbd/migration_target (5e2ed95ed806)
  81                 state: prepared
  82
  83 Note that the source image will be moved to the RBD trash to avoid mistaken
  84 usage during the migration process::
  85
  86     $ rbd info migration_source
  87     rbd: error opening image migration_source: (2) No such file or directory
  88     $ rbd trash ls --all
  89     5e2cba2f62e migration_source
  90
  91
  92 Execute Migration
  93 =================
  94
  95 After preparing the live-migration, the image blocks from the source image
  96 must be copied to the target image. This is accomplished by running the
  97 `rbd migration execute` command::
  98
  99     $ rbd migration execute migration_target
 100     Image migration: 100% complete...done.
 101
 102 The `rbd status` command will also provide feedback on the progress of the
 103 migration block deep-copy process::
 104
 105     $ rbd status migration_target
 106     Watchers:
 107         watcher=1.2.3.4:0/3695551461 client.123 cookie=123
 108     Migration:
 109                 source: rbd/migration_source (5e2cba2f62e)
 110                 destination: rbd/migration_target (5e2ed95ed806)
 111                 state: executing (32% complete)
 112
 113
 114 Commit Migration
 115 ================
 116
 117 Once the live-migration has completed deep-copying all data blocks from the
 118 source image to the target, the migration can be committed::
 119
 120     $ rbd status migration_target
 121     Watchers: none
 122     Migration:
 123                 source: rbd/migration_source (5e2cba2f62e)
 124                 destination: rbd/migration_target (5e2ed95ed806)
 125                 state: executed
 126     $ rbd migration commit migration_target
 127     Commit image migration: 100% complete...done.
 128
 129 If the `migration_source` image is a parent of one or more clones, the `--force`
 130 option will need to be specified after ensuring all descendent clone images are
 131 not in use.
 132
 133 Committing the live-migration will remove the cross-links between the source
 134 and target images, and will remove the source image::
 135
 136     $ rbd trash list --all
 137
 138
 139 Abort Migration
 140 ===============
 141
 142 If you wish to revert the prepare or execute step, run the `rbd migration abort`
 143 command to revert the migration process::
 144
 145         $ rbd migration abort migration_target
 146         Abort image migration: 100% complete...done.
 147
 148 Aborting the migration will result in the target image being deleted and access
 149 to the original source image being restored::
 150
 151         $ rbd ls
 152         migration_source
 153
 154
 155 .. _layered images: ../rbd-snapshot/#layering