SAS Hard Drives – Performance Differences Between 2.5″ and 3.5″

You should be concerned with head seek time and transfer rate. It is true that they depend on the form factor, but they also depend on many other variables. Looking only at the physical size and ignoring those variables would be wrong.

With this in mind, let's compare the most recent versions of some widely used disks: Seagate Cheetah 15K.7 and Savvio 10K.3.

• Random reads and writes: avg seek time of 3.4msec (read) and 3.9msec (write) vs. 4.2msec and 4.6msec, latency is 2.0 msec vs. 3.0msec. Therefore Cheetah should hit 185IOPS read and 170IOPS write, Savvio will do 140IOPS read and 130IOPS write.
• Sequential reads and writes: looking at sustained transfer rates, we see 122-204MBps for Cheetah and 67-124MBps for Savvio. These are maximums, the range is so wide because of different amount of data per track on the inside and the outside of the disk.

Bottom line: 2.5" disks are comparable to 3.5" disks for random operations, and usually you can compensate or even win with 2.5" due to the larger number of spindles in the same physical volume (disk array or server). However, if you need to pump a lot of sequential data, 3.5" disk is still the king.

No not really, 2.5 and 3.5 inch drives are mostly the same, including interface and whatnot but 3.5 inch drives are usually faster and larger in space because there is more stuff you can fit into 2.5 vs 3.5 inches. 3.5s are cheaper because of space constrains but usually no more reliable than a 2.5 and vise versa. (in practice)If you could fit a 3.5 drive do it, it'll be cheaper and bigger(storage wise)