What are the tradeoffs between using hypervisor- vs. storage-array-based snapshots as backup sources?

There are different ways to do it, below is an example how it is done using Commvault Simpana:

1. What happens when you backup VMs using VADP only

   • Backup software instructs vsphere to snapshot VMs
   • VMs are snapshotted
   • Backup software copies data using snapshots to backup media.It may also take advantage of other VADP features like CBT ( Change Block Tracking) to help speed up the process
   • VMs snapshots are released.

2. What happens when storage hardware snapshot is used

   • Backup software instructs vsphere to snapshot (also called quescing process) VMs. This is done to ensure VMs are in state ready for backup ( i.e. buffers are flushed etc).
   • Backup software instructs storage array to create hardware snapshot.This normally takes seconds.
   • Backup software instructs vsphere to delete VM snapshots.From this point, production VMs will not be affected and all backup processes will be done on snapshotted datastore.
   • Backup software instructs ESX server to mount hardware snapshot ( i.e datastore).When this is completed, VMs ( or rather their copies) are registered, the content indexed, if needed, and backed up to media using VADP method as described above.
   • Backup software instructs ESX server to unmount snapshotted datastore and also instructs array to delete hardware snapshot.

As you see , you have following advantages when using hardware snapshot:

• you only need to hold VM snapshots very short period - i.e untill all VMs you want to backup are snapshotted. Using first method however requires holding snapshots for entire backup time.
• you can make several VM backups during the day and keep them in hardware snapshot without copying data to media.

I think that your Dell storage supports VAAI, which includes copy offloading. I don't claim to be an expert on VMWare or Dell storage, but I found a page that explains it from VMWare's perspective. I know that full copies can be offloaded to the storage because we do that here with an HDS. They say that VM Snapshots can be offloaded to the storage, but I haven't ever tried it.