Quadric Software

Automating leaf coalesce to reclaim snapshot storage

Please note: this article relates to Alike v3.x and earlier. Due to limited use of this feature in modern XenServer versions, this feature was deprecated as of Alike v4.0

In some versions of XenServer (5.5-6.5), when a snapshot is deleted, the space consumed by the snapshot is not always fully reclaimed, depending on your SR configuration. This is documented with the description from Citrix's XenServer 5.5 release notes of issue CA-30049. Because Alike's Enhanced Backup/Replication jobs leverage the snapshot functionality in XenServer, you may first become aware of this issue when using Alike. 

As of XenServer 5.5 Update 1, Citrix provided a manual workaround for this issue, which must be run on Dom0, called a 'leaf coalesce'. It will suspend the effected guest and free the extra space that was consumed by the snapshot, then resume it. In later versions of XenServer, this process has mostly been automated, and is performed in the background by the hypervisor, making manual leaf-coalescing largely unnecessary. However, occasionally leaf-coalescing may still be needed for some environments.

With Alike, you can automate a leaf-coalesce as part of a backup job. You may also choose to schedule just a leaf-coalesce job on its own, if you prefer.  To run a leaf-coalesce job, simply create a new job, and choose the 'leaf-coalesce' job type. Then proceed to add all the VMs/Hosts you wish to include.  To run the leaf-coalesce as part of a backup job, simply check the box "Coalesce All Guests After Job" on Step 4 of the Backup Wizard. This will cause Alike to trigger a leaf-coalesce after the backup portion of the job is complete.

Warning: the Leaf Coalesce script must temporarily suspend each Guest in order to reclaim the space. While suspended, these VMs will be briefly inaccessible to requests.

Please Note: Leaf-coalesce jobs are only available for XenServer platforms. Leaf-coalescing is not necessary for, or provided by hypervisor platforms, or physical systems.