During the snapshot removal step of a Veeam job, the source VM loses connectivity temporarily.
Veeam does not remove the snapshot itself, Veeam sends an API call to VMware to have the action performed.
The snapshot removal process significantly lowers the total IOPS that can be delivered by the VM because of additional locks on the VMFS storage due to the increase in metadata updates, as well as the added IOP load of the snapshot removal process itself. In most environments, if you are already over 30-40% IOP load for your target storage, which is not uncommon with a busy SQL/Exchange server, then the snapshot removal process will easily push that into the 80%+ mark and likely much higher. Most storage arrays will see a significant latency penalty once IOP's get into the 80%+ mark which will of course be detrimental to application performance.
The following test should be performed during a time when connectivity to the VM is not sensitive, for instance during off peak hours.
To isolate this issue to the specific VMware snapshot removal event, Veeam suggests the following isolation test:
If while performing the test above you observe the same connectivity issues as during the Veeam job run, the issue very likely exists within the VMware environment itself. Please review the following list of troubleshooting steps and known issues. If none of the following work to resolve the issue, we advise that you contact VMware support directly regarding the snapshot removal issue.
Note: This issue will present as multiple minutes worth of stun. Normal snapshot stun is only mere seconds.
At the time of the writing of this KB there is a known issue with NFS Datastores and Virtual Appliance (HOTADD) transport mode. The issue is documented in this VMware KB article: http://kb.vmware.com/kb/2010953
Veeam advises that if this issue occurs one of three things can be done to work around this:
1. Use Direct NFS Mode
More details available here:
2. Configure Veeam Backup & Replication to use a Proxy on the same host as the VM
Note: This option is only available in versions newwer than 188.8.131.529
Note: Both values 1 or 2 will enable the SameHostHotaddMode, which forces Veeam B&R to first attempt to use the Proxy that is on the same host as the VM to be backed up.
3. Reconfigure proxies to use Network Transport mode.
The following links are provided to help understand more about the Snapshot Removal process within VMware: