I’d never heard of disk IOPS (Input/Output Operations Per Second) until I started researching what kind of SAN we need to by for our virtualisation project.
But I went to a seminar by an HP storage expert who pointed out that since the days when I started working in IT (late eighties), disk capacity has increased by an eye-watering 10,000 times but disk performance has only increased a measly fifty times. So squeezing out performance from your disks is A. Big. Deal. So it becomes necessary for an IT Manager like me to roll up his sleeves and learn about IOPS.
The HP P2000 SAN that I’m looking at comes in two versions, a 12 bay model supporting LFF disks (3.5 inch disks) and a 24 bay model supporting SFF disks (2.5 inch disks). They’re both about the same price. So the 24 bay looks more attractive, right? The problem is SFF disks only spin at 10k RPM whereas the LFF disks spin at a much faster 15k RPM.
My reseller has recommended against buying SFF disks because he says the performance won’t be adequate for our needs. He’s recommended twelve 300GB LFF disks in a RAID 6 array. This gives 3TB of usable data. Not bad, but it fills all the bays so there is no room for expansion if we ever need it (well, there is, but it requires purchasing another drive shelf which isn’t cheap).
But I suspect that talk about spin speeds is over-simplifying the problem somewhat. The number of disks you have, and the RAID level you use, will all effect performance. Where performance is counted by IOPS. Its average IOPS that define performance, not RPM.
With the 24 bay P2000, twenty 300GB SFF disks in a RAID 10 array will give the same amount of usable data. It also leaves four bays free, so we can expand our storage later on by upto 20% – a nice buffer.
Which is better? It all depends on the IOPS. I have no idea. But given that RAID10 offers much better performance than RAID6, it wouldn’t surprise me if those “slow” SFF disks outperform the “fast” LFF disks. I could work it out with a fancy Excel spreadsheet, but my understanding of the inputs is very limited. I’m hoping someone will do it for me.
Obviously having to buy twenty disks instead of twelve will up the budget somewhat (by just under two grand). But then the argument becomes “choose LFF disks because its cheaper” and not “choose LFF disks because of the performance”. When a disk fails and I’m waiting for an array to rebuild then I’m stressed out (its the pessimist in me). RAID6 is better than RAID5 in that it can survive two disk failures, but I’d still rather pay an extra couple of grand to get the resilience of RAID10 (its possible we could have ten disks fail and still not lose our data). Because RAID6 only survives two disk failures, it doesn’t obey the law that states “bad news comes in threes”.
The other reason for choosing SFF disks over LFF is that they’re just so much sexier. I know size isn’t supposed to be important, but it is. 24 little disks in a tiny 2U bay, who couldn’t get turned on by that?