It all depends really on the usage. If you're using a database and IOPS is 
important, you'll see a significant increase. If you're running a backup 
storage array, capacity/$ is important.

SAS/SCSI hard drives at 15000rpm are of course a bit faster than 7200rpm 
SATA/SAS/SCSI drives but if your network is a bottle neck, it doesn't really 
matter whether you put in 5400rpm drives or SSD's.

Also with RAID (or RAID-like systems like ZFS) and a lot of hard drives, the 
bottle neck really shifts towards the controllers and/or the bus it's connected 
to. So you should analyze why the decision was made to use SATA disks (usually 
price) and whether the cost of SAS drives (300% increase in costs, 50% increase 
in speed, 300% decrease in potential capacity) is really worth it.

Benchmark your system and the disks you're looking at and see whether it's 
worth it. IMHO: for most uses not, attaching/enabling caches or getting more 
memory could increase your responses significantly for way less.
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