As far as I know, there still exists no sane backup strategy for multiple filers, unless you think having one tape library per filer or backing up multiple terabytes of data over the network and through some kind of backup management box is sane.
You don't need a tape library per filer if you use NDMP compliant backup software. NDMP supports SCSI pass-through so that the backup software can co-ordinate tape loading activities across multiple hosts mounted on multiple drives, filers included. You only need one drive per filer.
With three-way NDMP, you don't even need a dedicated drive per filer, but the filers would be chatting over one of the networks between them. On the other hand, that could be a dedicated link if you want.
The key to NDMP is that the backup management box doesn't need to touch the data. It handles scheduling, indexes, and logs, but otherwise just tells hosts to back up to either their own local drives or some other NDMP-compliant host's drives.
You should never need to resort to a library per filer.
Regards, Garth.
-- Garth T Kidd garth@netapp.com Mobile: +61-411-596-593 Consulting Systems Engineer, Asia Pacific Mobile Fax: +61-3-9228-9732 Network Appliance http://www.netapp.com/