Hello, Actually, according to media.netapp.com/.../tr-3702.pdf which has last been updated July 2011, the NetApp best practice is 0% space reservation. I believe 20% is the default of a new volume but there are very few defaults on NetApp which are also recommended practices.  In general, the netapp best practices revolve around setting few reservations on volumes, but creating AutoDelete policies and AutoGrow policies that keep your growth in check by deleting older snapshots.  This works well for keeping everything thin + deduplication. We use EMC NetWorker for backup, but the VSS snapshot as created by the backup application interacting with VSS hw provider does get deleted at the end of the backup.  The snapshot is to backup open files in a consistent manner. For the snapshot and volumes to stay online, you need enough reserve so that the change rate does not exceed the reserve for the duration of the snapshot.  You could set a conservative reserve like 20% and watch how much remains at the end of the backup.
I plan to use DPM 2010 for backing up my Hyper V hosts on EQL storage.  Currently I have some volumes that I have set the snap reserve to 20% (2 TB LUN's).  These are CSV volumes used by Hyper V.  I dont plan on using ASM or any kind of direct storage snapshot schedule.....just DPM.   After reading the DPM documentation it states that if the SAN vendor provides a Hardware VSS agent, which I believe comes with the Hit Kit (3.51 on my Hyper V hosts) DPM will instruct the storage via the hardware VSS provider to create a snapshot so DPM can back it up.  I would assume its much faster than a software snap shot created by DPM? Anyhow my questions are...     Thanks for any feedback!   -Lindy
Snapshot space.