There are a variety of inputs when considering the number of drives.  Here's
one:

Total newly arriving data * 2 (for copy pools) / data rate per drive.

So, let's say you have 100 GB of data per day.  Times 2 is 200 GB.  Let's
say we can write 10 GB per drive per hour and further let's say the time
available to write this data is 8 hours.  We need 20 hours of drive time.
Two drives won't be enough so three is the answer.  I like to have a drive
around for the odd restore, so in this case, I would probably configure four
drives.

Oftentimes, you will have more than 8 hours during the day to write the data
from the diskpools to the copy and tape pools.

How much data do you have?

Kelly J. Lipp
Storage Solutions Specialists, Inc.
PO Box 51313
Colorado Springs CO 80949-1313
(719) 531-5926
Fax: (240) 539-7175
Email: lipp AT storsol DOT com or lipp AT storserver DOT com
www.storsol.com
www.storserver.com