Relevant mostly to OS X admins
Category Archives: Virtualization
June 8, 2013Posted by on
I’ve administered a small pool of 2 physical XenServers with shared storage for my employer’s Windows and Linux virtual servers for a few years. One issue that has come up under both XenServer 5.6 and 6.x is a failure for XS to properly remove the disk files when a snapshot is deleted. This can be an issue when using a VM backup tool such as PHDVirtual, where the backup server is another VM, which on a schedule has the hypervisor
- Take a snapshot of another running VM
- Attach that snap to the backup VM
- Read that drive, back up to the configured storage
- detach and delete the snap
Result is a buildup of undeleted snapshot files, taking up storage space on your SR. One way to confirm if this is happening is to execute the following line on the XenServer console:
xe vdi-list is-a-snapshot=true | grep name-label | sort
If you don’t expect to see any snapshots, seeing them listed here is an issue.
These shouldn’t be here.
Citrix offers a command line tool to address this, outlined in Knowledgebase document CTX123400. This is called an Offline Coalesce, and is formatted as
xe host-call-plugin host-uuid=<UUID of the pool master Host> plugin=coalesce-leaf fn=leaf-coalesce args:vm_uuid=<uuid of the VM you want to coalesce>
Earlier this week, I was using the above in hopes to address the large discrepancies in my SR used vs allocated values from the not deleted, but not seen in XenCenter snapshots seen above:
Problem is, it didn’t work. Tailing /var/log/SMlog, the standout clue was “no space to coalesce”. So great… a bug in snapshots takes up all your space, and when you try to use the tool to fix it, it can’t because there’s no space.
A day later, this became quite concerning. I don’t know what Xen will do when an SR hits 100% capacity, but I doubt I’ll enjoy it.
I decided to take a gamble and see what happens if I move a disk to a different SR. Lacking Storage XenMotion, I shut down a non-critical VM- the Windev2 machine listed above, detached the C: drive, and asked Xen to move it to a different SR. It took much longer than expected (about 30 minutes for 48 gigs), but in the end, I regained much more than 48 gigs of storage on my SR:
557.6- 281.9= 275 gigs of wasted snapshot space due to XenServer bugs, restored by moving the disk to a different SR. My nightly PHDVirtual backups were now able to take a snapshot last night and perform a proper backup.
- When leaf-coalesece fails, moving a disk to different storage can clean up the wasted storage space
- If I had to do it all over, I think I’d go VMWare. XenServer has had problems with snapshot management for a long time, and they still haven’t figured it out.