On Tue, Aug 4, 2009 at 10:33 AM, Richard Elling<richard.ell...@gmail.com> wrote: > On Aug 4, 2009, at 7:01 AM, Ross Walker wrote: >> >> On Aug 4, 2009, at 7:26 AM, Joachim Sandvik <no-re...@opensolaris.org> >> wrote: >> >>> >>> does anybody have some numbers on speed on sata vs 15k sas? Is it really >>> a big difference? >> >> For random io the number of IOPS is 1000/(mean access + avg rotational >> latency) (in ms) >> >> Avg rotational latency is 1/2 the rotational latency which is 1/(rotations >> per second), which for 15K = 250 and for 7200 = 120. >> >> This means for 15K drives the arl = 2ms and for 7200 the arl = 4.150ms. >> >> So for each random io take the mean access time add the arl and divide >> 1000 by that number. >> >> SAS disks tend to have faster access times, so say a top SAS disk has an >> access time of 4ms + ARL = 6ms, a top SATA disk has an access time of 8ms + >> ARL = 12ms > > Unfair! You are comparing 2.5" drives to 3.5" drives. The seek times > depend on the size (diameter) of the disk. Smaller diameter == faster > seek. Where you have to be careful is that laptop drives tend to be 2.5" > but they also run slower, 5,400 rpm. 1 TB 2.5" drives are beginning to > appear, but it looks like they will be slower -- targeted at laptops, not > servers.
It was merely an example of how to calculate the IOPS of a given drive and to show the OP the kind of speed 15K SAS affords over 7200 SATA. You must plug in your drive's own seek numbers. You are fooling yourself though if you think the size makes a difference in the seek times. Take a look at the technical specifications. Average Latency = Average Rotational Latency, Average Seek Time = Mean Access Time, when looking at the specs. >From what I have read online my example is very optimistic. The platters are getting smaller, but the linear density is increasing and the heads are getting smaller. It's like a 3.5" drive, but shrunk in proportion. -Ross _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
