When you size storage, you have to size for three things: IOPS, capacity and bandwidth. But that has an interesting implication. When we had 2,4 or 9 GB harddisks we needed many disks to provide the needed capacity and thus had often enough IOPS for the tasks of that time without needing further provisions. Then disks got bigger, and we created massive storage arrays with much to large capacity , but we did it for the IOPS. Now we have SSD in the TB range and with IOPS north of 100.000 and you would think that your problems are gone, however now the third parameter comes into the game: Bandwith.
So far you had often enough disks (either because of bandwidth or IOPS requirements) and thus enough raw bandwidth at least at the point right behind the disk). However it doesn't matter, when you have 1 TB and 100.000 IOPS, you still have just a storage connection that limits your transmission at 6 GBps, come hell or high water. So after explaining "No, a 1 TB USB disk from el-cheapo-component-shack isn't enough for the 1 TB database, because of IOPS yadda yadda yadda" to the people in the purchasing department, we probably have to start to explain "No, the 1 TB SSDs aren't enough as well, because of the bandwidth requirements yadda yadda yadda"
IOPS, capacity, bandwidth - and something new to explain to the people in purchasing http://www.c0t0d0s0.org/archives/7464-IOPS,-capacity,-bandwidth-and-something-new-to-explain-to-the-people-in-purchasing.html