VM Loses Connection During Snapshot Removal

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 when connectivity to the VM is not sensitive, for instance, during off-peak hours.

To isolate the  VMware snapshot removal event, Veeam suggests the following isolation test:

1. Create a snapshot on the VM in question.
2. Leave the snapshot on the VM for the duration of time that a Veeam job runs against that VM.
3. Remove the snapshot.
4. Observe the VM during the snapshot removal.

While performing the test above, if you observe the same connectivity issues as during the Veeam job run, the issue likely exists within the VMware environment itself. 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.

• If the environment is using NFS 3.0 Datastores, see the section below about a Known Issue with NFS 3.0 and Hotadd.
• Check for snapshots on the VM while no Veeam job is running and remove any that are found.
  
  Veeam Backup & Replication is able to back up a VM that has snapshots present, however, it has been observed that when VMware attempts to remove the snapshot created during a Veeam job operation and there is a snapshot already present on the VM, snapshot stun may occur.
  
• Check for orphaned snapshots on the VM. (See: http://kb.vmware.com/kb/1005049)
• Reduce the number of concurrent tasks that are occurring within Veeam, this will in turn reduce the number of active snapshot tasks on the datastores.
• Move VM to a datastore with more available IOPS, or split the disks of the VM up into multiple datastores to more evenly spread the load.
• If the VM's CPU resources spike heavily during Snapshot consolidation, consider increasing the CPU reservation for that VM.
• Ensure you are on the latest build of your current version of vSphere, hypervisors, VMware Tools, and SAN firmware when applicable.
• Move VM to a host with more available resources.
• If possible, change the time of day that the VM gets backed up or replicated to a time when the least storage activity occurs.
• Use a workingDir to redirect Snapshots to a different datastore than the one the VM resides on. http://kb.vmware.com/kb/1002929

This issue will present as multiple minutes worth of stun. Normal snapshot stun is only mere seconds.

There is a known issue with NFS 3.0 Datastores and Virtual Appliance (HOTADD) transport mode. The issue is documented in this VMware KB article: http://kb.vmware.com/kb/2010953. "This issue occurs when the target virtual machine and the backup appliance [proxy] reside on two different hosts, and the NFSv3 protocol is used to mount NFS datastores. A limitation in the NFSv3 locking method causes a lock timeout, which pauses the virtual machine being backed up [during snapshot removal]."

If this issue occurs, you should implement one of three following solutions:

For more information about "Snapshot Stun" please review this VMware KB artcile, Snapshot removal stops a virtual machine for long time (1002836)