Backup fun-n-games

I run Citrix XenServer in our data center for virtualization. In general, I'm pleased with it but have run into a big problem with the way space is not reclaimed during snapshots. It seems that when a snapshot is deleted, all of the space is not freed up when are using iSCSI storage. This is a big problem because you eventually run out of disk space and can't create any more snapshots to backup.

I've tried following the Citrix solution to reclaim space, but it doesn't seem to work. The results are always something like:

coalesce-leaf -u 36b672b6-302e-0b2a-84f1-9b74f00f88f7
VDI to consider: e9915854-773a-4c22-a873-5697a5561add
VDI to consider: 47e9730d-66d1-4e04-94fa-c64f43c8de27
Processing VDIs: ['e9915854-773a-4c22-a873-5697a5561add', '47e9730d-66d1-4e04-94fa-c64f43c8de27']
Finishing regular GC/coalesce work for SR 209f ('iSCSI nearline disk1')...
Failed:

I've managed to clear up about 200GB of space by running the coalesce-leaf command and deleting out some VMs that I don't need any more. Plan B is to move everything off of that iSCSI unit and move it back in. Not fun, but I think that's going to have to happen over the weekend if the backups don't run tonight.