Currently cinder supports volume migration with backend assistance. Some backends like IPSANs support migrating volume in backend level, but Ceph does not. Though Ceph volume migration has already been supported through cinder generic migration process in Liberty, the migration efficiency is not so good. It is necessary to support volume migration in the Ceph driver to improve the efficiency of migration.
Suppose there are two cinder volume backends which are in the same Ceph storage cluster.
Now volume migration between two Ceph volume backends is implemented by generic migration logic. Migration operation can proceed with file operations on handles returned from the os-brick connectors, but the migration efficiency is limited on file I/O speed. If we offload migration operation from cinder-volume host to Ceph storage cluster, we would get the following benefits:
There are three cases for volume migration. The scope of this spec is for the available volumes only and targets to resolve the issues within the following migration case 1:
Within the scope of this spec:
1.Available volume migration between two pools from the same Ceph cluster.
Out of the scope of the spec:
2.Available volume migration between Ceph and other vendor driver.
3.In-use(attached) volume migration using Cinder generic migration.
Solution A: use rbd.Image.copy(dest_ioctx, dest_name) function to migrate volume from one pool to another pool.
To offload migration operation from cinder-volume host to ceph cluster, we need to do the following changes in migrate_volume routine in RBD driver.
Solution B: use ceph’s functions of clone image and flatten clone image to migrate volume from one pool to another pool.
Solution B contains the following steps: * Create source volume snapshot snap_a and protect the snapshot snap_a. * Clone a child image image_a of snap_a to the destination pool. * Flatten the child image image_a, thus snap_a has not been depended on. * Unprotect the snapshot snap_a and delete it.
Using a volume which’s capacity and data size is 12GB to show the time-consuming comparison between solution A and solution B.
[Solution-A]Copy volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52 from “volumes1” pool to “volumes2” pool.
root@2C5_19_CG1# time rbd cp volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52 volumes2/test1
Image copy: 100% complete...done.
real 2m3.513s user 0m9.983s sys 0m25.213s
[Solution-B-step-1]Create a snapshot of volume 777617f2-e286-44b8-baff-d1e8b792cc52 and protect it.
root@2C5_19_CG1# time rbd snap create –pool volumes1 –image volume-777617f2-e286-44b8-baff-d1e8b792cc52 –snap snap_test
real 0m0.465s user 0m0.050s sys 0m0.016s
root@2C5_19_CG1# time rbd snap protect volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52@snap_test
real 0m0.128s user 0m0.057s sys 0m0.006s
[Solution-B-step-2]Do clone operation on volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52@snap_test.
root@2C5_19_CG1# time rbd clone volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52@snap_test volumes2/snap_test_clone
real 0m0.336s user 0m0.058s sys 0m0.012s
[Solution-B-step-3]Flatten the clone image volumes2/snap_test_clone.
root@2C5_19_CG1# time rbd flatten volumes2/snap_test_clone Image flatten: 100% complete...done.
real 1m58.469s user 0m4.513s sys 0m17.181s
[Solution-B-step-4]Unprotect the snap volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52@snap_test and delete it.
root@2C5_19_CG1# time rbd snap unprotect volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52@snap_test
real 0m0.150s user 0m0.058s sys 0m0.013s
root@2C5_19_CG1# time rbd snap rm volumes1/volume-777617f2-e286-44b8-baff-d1e8b792cc52@snap_test
real 0m0.418s user 0m0.054s sys 0m0.011s
By the above test results of solution A and solution B, solution A needs (real:2m3.513s, user:0m9.983s, sys:0m25.213s) to finish the volume copy operation and solution B needs (real:1m59.966s, user:0m4.790s, sys:0m17.239s) to do that. The time-consuming for the both two solutions are not much difference, but solution A is more simpler than solution B. So we intend to use solution A to offload volume migration from cinder-volume host to ceph cluster.
The performance of volume migration between two Ceph storage pools in the same Ceph cluster will be improved greatly.
Add location info of back-end:
Add location_info in state of Ceph volume service, it should include the Ceph cluster name(or id) and storage pool name.
Implement volume migration:
1.Check whether the requirements of volume migration are met. If source back-end and destination back-end are in the same Ceph cluster and volume status is not ‘in-use’ state, the volume can be migrated.
2.Copy volume from one pool to another pool and keep it’s original image name.
3.Delete the old volume.
Unit tests will be added. Volume migration test case will be added.
Both unit and Tempest tests need to be created to cover the code change that mentioned in “Proposed change” and ensure that volume migration feature works well while introducing the feature of RBD volume migration.