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

1. Take a snapshot of another running VM
2. Attach that snap to the backup VM
3. Read that drive, back up to the configured storage
4. 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