Fortunately for me, someone else here takes care of backups, but...
We have a similar situation on our F740. We want to keep the volume size small because: a) Restores can take forever as the data size gets bigger.
Can you not back up with qtree granularity instead of per volume?
b) 10-14 disks is a good compromise on reliability. Having 2 out of 14 disks go bad at one time is much rarer than 2 out of 51 disks.
You can beat this by using several raid groups per volume. Of course, you need a parity drive for each raid group.
c) If you upgrade disks/shelves in the future, you will likely do it a volume at a time. We did this with a volume with 180GB and the volcopy took 6-7 hours to complete. It is NOT very fast. With 400GB+ in a volume, that should be 2-2.5x longer.
Hmm. My data-shuffling tends to be in smaller pieces. Wouldn't it be nice to have a qtreecopy? 7 hours for 180 GB sounds like about 7 MB/second. If this is on 100 MB, it's not all that bad.
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Louis