Do the Q&A documents you reviewed include the "Oracle ZFS Storage: FAQ: Exadata RMAN Backup with The Oracle ZFS Storage Appliance (Doc ID 1354980.1)" note? This note has lots of information and is updated regularly and includes information on ZFS configuration and performing RMAN backups to the ZFS appliance.
A couple of useful white papers, one I have used in the past and was updated again last year: Protecting Oracle Exadata with the Sun ZFS Storage Appliance: Configuration Best Practices and Backup and Recovery Performance and Best Practices using Sun ZFS Storage Appliance with Oracle Exadata Database Machine
Those documents go into a lot of detail - including giving sample scripts.
go through the pdf and you may find the solution.
although its too lengthy:
Or you may find something useful in
Oracle ZFS Storage: FAQ: Exadata RMAN Backup with The Oracle ZFS Storage Appliance (Doc ID 1354980.1)
I would also recommend ensuring you are using dual active controllers for the best backup speeds. However as one share can only reside on one controller it makes best sense if you have multiple databases to be backed up.