Scenario: We have a VM running on an ESXi5.1 host that was taking over an hour to backup via Veeam. All the others were taking considerably less.
During the next run of the scheduled Veeam backup it seemed that the slow-down occurred when removing the snapshot Veeam creates. Having a browse of the datastore and then selecting the particular VM folder I noticed there were 2 snapshots (when I was only expecting 1)
I waited until the Veeam backup finished then looked at the datastore again
Upon inspection it showed one snapshot file and looking at the date, it was created over 2 months ago and was a considerable size. In ESXi5.0 and below I actually setup an alert to notify the administrators when a VM was running on a snapshot but Esxi5.1 has a known bug ( KB2037464 ) so that feature had to be turned off while VMware find a fix.
I immediately deleted this snapshot and then checked the datastore:
All looks good now. one vmdk file and one snapshot file.
The next backup worked in normal time.