Automating leaf coalesce to reclaim snapshot storage
When a snapshot is taken and subsequently 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 our product leverages the snapshot
functionality in XenServer to perform backups, you may
first become aware of this issue when using Alike.
On December 15th, 2009, Citrix released XenServer 5.5 Update 1, which contains a manual workaround tool for this issue that runs on Dom0 called a 'leaf coalesce'. It will suspend the effected guest and reclaim the extra space that was consumed by the snapshot, then resume it. Update 1 is available here.
As of Alike v1.31, released on 12/18/09, you can automate leaf coalesce as part of a backup job. You may also choose to schedule coalesces without backing up any guests as a separate procedure. Do not apply run leaf coalesce unless your environment is affected by the issue, and make sure you understand the implications of the leaf coalesce before you proceed!
On Step 4 of the Backup Wizard, you will see two checkboxes below the job summary. The checkbox marked "Coalesce All Guests After Job" will enable the Leaf Coalesce script for each Guest after the Job is complete. Warning: the Leaf Coalesce script must temporarily suspend each Guest in order to reclaim the space. Suspended guests are offline!
Alternatively, you can schedule a Job that only runs a coalesce, and doesn't conduct backups. On Backup Wizard Step 1, select the "Leaf Coalesce" job type, and then select which guests you'd like to coalesce on the subsequent wizard screen.
Use the first approach if you are okay with suspending your guests after the job has run. Use the second approach if you would like to dedicate a daily, weekly, or monthly Job that is completely distinct from your normal backup activities to reclaim snapshot space. This second approach is useful because it allows you to plan for your guest outages as distinct from any other backup activity.