Hi Dominic, some answers:
1. there is a limit of concurrent snapshots per datastore when using regular backups, but if you are talking about BfSS, this limit does not apply and we try to consolidate all VM snapshots in that datastore before taking the storage snapshot. There are so no best practices on this other than what you prefer to use. Just don't group too many VMs into the same job belonging to the same datastore, otherwise the job will wait for all vsphere snapshots to be created before the storage snapshot
2. not relevant to NetApp, it's more for deduplication appliances
3. in both cases, vpowerNFS is executed in the configured repository. I'm not sure I've understood the part related to vmdk: the backup file is always mounted from the vpowerNFS service, and exposed as a simulated datastore to the hypervisor, regardless the backup file is stored in a local repository or a CIFS share.
4. you mean when using a NetApp machine as a target? LArger block sizes have better performances because Veeam proxies have to process fewer larger blocks, and also in terms of writing those to the final destination, it means a single write will write a larger block to the storage.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1