When you refer to Aux Copy, is this:
- Aux Copy from Local to Cloud - if this is the case Local should be using 128 KB Block Size for DDB, and Cloud DDB should be using 512 KB.
- Aux Copy from Local Disk to Disk - both DDB should be using 128 KB
- Depending on the type of Storage Array (i.e Isilon, NetApp, Synology, HDS and etc) there will be specific recommendation
- Aux Copy from Local Disk to Tape - this will depend on the Read from Source Storage, Network and the Physical Write to Tape (no Deduped involved)
If you are currently using Encryption, you can consider using the option "Preserve Encryption from Source" which will mitigate from the need of re-encrypting the data when writing to Secondary.
Commvault will not allow you to share the same DDB for two copy in the same Storage Policy, as that is a single Point of Failure (and even if Commvault allows you to do so, you will never configure it that way).
If you have Deduplication on Source and Destination, you can enable DASH Copy, which is only comparing the unique signature on Source and transfering/incrementing the Seocndary DDB (which will greatly improve overall throughput/completion time as there are less data to transfer over the wire).
There are various factors to consider, but if you can share your specific scenario, we can assist or provide more recommendation inline to Commvault Best Practice.