IOPS

1. a user generally asks for,
       400 gb capacity
       1000 iops with 30% writes, 70% reads
       raid 5 protection (decided)

2. storage admin calculates required iops for    r-5(3+1) writes and reads.
   for r-5, write penalty is 4. (2 reads, 2 writes)
 
   as 30% writes, 300 iops*4 = 1200 iops
      70% reads,  700 iops   =  700 iops
                               ----------
                               1900 iops

3. if you have 146 gb drives with 200 iops, then
   for assigning 400gb,
       you can take = 3*146gb = 400gb
   but total iops are = 3 * 200 iops = 600 iops
 
 But the requriement is 1900 iops.

so, you require 1900/200 = 10 drives.

for raid-5 (3+1), the drives must be multiple of 4.
=> so, 12 drives are needed for that particular raid group.