Image Live-Migration

RBD images can be live-migrated between different pools within the same cluster or between different image formats and layouts. When started, the source image will be deep-copied to the destination image, pulling all snapshot history and optionally keeping any link to the source image’s parent to help preserve sparseness.

This copy process can safely run in the background while the new target image is in-use. There is currently a requirement to temporarily stop using the source image before preparing a migration. This helps to ensure that the client using the image is updated to point to the new target image.

Note

Image live-migration requires the Ceph Nautilus release or later. The krbd kernel module does not support live-migration at this time.

The live-migration process is comprised of three steps:

  1. Prepare Migration: The initial step creates the new target image and cross-links the source and target images. Similar to layered images, attempts to read uninitialized extents within the target image will internally redirect the read to the source image, and writes to uninitialized extents within the target will internally deep-copy the overlapping source image block to the target image.

  2. Execute Migration: This is a background operation that deep-copies all initialized blocks from the source image to the target. This step can be run while clients are actively using the new target image.

  3. Finish Migration: Once the background migration process has completed, the migration can be committed or aborted. Committing the migration will remove the cross-links between the source and target images, and will remove the source image. Aborting the migration will remove the cross-links, and will remove the target image.

Prepare Migration

The live-migration process is initiated by running the rbd migration prepare command, providing the source and target images:

$ rbd migration prepare migration_source [migration_target]

The rbd migration prepare command accepts all the same layout optionals as the rbd create command, which allows changes to the immutable image on-disk layout. The migration_target can be skipped if the goal is only to change the on-disk layout, keeping the original image name.

All clients using the source image must be stopped prior to preparing a live-migration. The prepare step will fail if it finds any running clients with the image open in read/write mode. Once the prepare step is complete, the clients can be restarted using the new target image name. Attempting to restart the clients using the source image name will result in failure.

The rbd status command will show the current state of the live-migration:

$ rbd status migration_target
Watchers: none
Migration:
            source: rbd/migration_source (5e2cba2f62e)
            destination: rbd/migration_target (5e2ed95ed806)
            state: prepared

Note that the source image will be moved to the RBD trash to avoid mistaken usage during the migration process:

$ rbd info migration_source
rbd: error opening image migration_source: (2) No such file or directory
$ rbd trash ls --all
5e2cba2f62e migration_source

Execute Migration

After preparing the live-migration, the image blocks from the source image must be copied to the target image. This is accomplished by running the rbd migration execute command:

$ rbd migration execute migration_target
Image migration: 100% complete...done.

The rbd status command will also provide feedback on the progress of the migration block deep-copy process:

$ rbd status migration_target
Watchers:
    watcher=1.2.3.4:0/3695551461 client.123 cookie=123
Migration:
            source: rbd/migration_source (5e2cba2f62e)
            destination: rbd/migration_target (5e2ed95ed806)
            state: executing (32% complete)

Commit Migration

Once the live-migration has completed deep-copying all data blocks from the source image to the target, the migration can be committed:

$ rbd status migration_target
Watchers: none
Migration:
            source: rbd/migration_source (5e2cba2f62e)
            destination: rbd/migration_target (5e2ed95ed806)
            state: executed
$ rbd migration commit migration_target
Commit image migration: 100% complete...done.

If the migration_source image is a parent of one or more clones, the –force option will need to be specified after ensuring all descendent clone images are not in use.

Committing the live-migration will remove the cross-links between the source and target images, and will remove the source image:

$ rbd trash list --all

Abort Migration

If you wish to revert the prepare or execute step, run the rbd migration abort command to revert the migration process:

$ rbd migration abort migration_target
Abort image migration: 100% complete...done.

Aborting the migration will result in the target image being deleted and access to the original source image being restored:

$ rbd ls
migration_source