Evan Unrue - Product Specialist at Magirus UK
Apart from the obvious space-saving benefits of having a smaller form factor drive – which would allow for higher density of disks, consuming less physical footprint when considering RAID arrays – 2.5-inch drives do appear to have some performance gains over their bigger 3.5-inch brother when looking at manufacturers' published specifications.
Full stroke and track-to-track seek times on 2.5-inch drives come out at near half that of 3.5-inch drives in most cases, which one can only assume is down to the smaller platter and actuator arm. This would clearly have benefit in systems with random access patterns to disk. That said, there is an argument to say that 3.5-inch disks would potentially see higher performance in systems with highly sequential access patterns to disk due to the age old marvel that is zoned bit recording – as pragmatically speaking we should have more sectors on the outer tracks of a 3.5-inch drive than a 2.5-inch drive due to the larger platter.
With 3.5-inch disks, more sectors would pass under the read/write heads of the disk, resulting in a higher data read/write rate. However, this benefit would only be seen if data written/read to or from disk was done so in a purely sequential fashion and that typically has very specific use cases such as media streaming, backup to disk and the like (not what we would consider a general workload). Essentially 2.5-inch drives would see slower transfer speeds than 3.5-inch drives, but faster access times.
With all that being said, we are in an age of miniaturisation. Things which come in smaller packages typically are less power hungry. From what I’ve seen a 2.5inch drive will typically consume 40 per cent less power than a 3.5-inch drive of comparable rotational speed.
When looking at the enterprise storage vendors, one of the elements which appears to have let 3.5-inch drives hang in there is the lingering of Fibre Channel drives in the market, which I don't believe helped the introduction of the smaller form factor into the mainstream. Now 6Gbit/s SAS drives are outed, it looks like Fibre Channel drives, which only support a 4Gbit/s interface speed may get the chop, clearing the way for SAS and SATA drives across the board moving forward (one would think), and potentially allowing for a standardised smaller form factor in the enterprise storage arena.