SATA IOPS Measurement

Question to the Storage Advisors, from Michael K.: I’ve heard great things about SATA based DASD and JBOD devicess. I’ve found tons of information about the data transfer rate, but haven’t been able to find any hard data on how many sustained and bursted IOPS such systems can handle.

Michael, IOPS are very dependent on the access pattern. For this reason it’s normal to measure the “worse case” IOPS which are acheived on short random IOs. The IOPS can be calculated by simply adding the time to do an average seek to the time for a half rotation. Note that the transfer rate doesn’t come into play because it’s so insignificant compared to the seek and rotation time.

For example, the 500GB Western Digital SE16 SATA drive (WD5000KS) has a rotational rate of 7200 RPM. By inverting that number you get one rotation per 8.4ms. Therefore a half rotation is 4.2ms. This is also noted on the spreadsheet as “Average Latency”.

Next, the average seek times for reads are listed as 8.9ms, while writes are 10.9ms. That difference may seem odd at first, but drives eek out a little extra performance on reads by attempting to read the data before the head has settled on the track (and that settling for writes takes ~2ms). So, let’s say that you have an even mix of reads and writes, and therefore we’ll assume the average seek time is 9.9ms.

Adding 4.2ms and 9.9ms gives has an average random IO latency of 14.1ms. Since the drive can process, i.e., access the media, with just one command at a time, you can invert 14.1ms to get 71 IOPS.

You can start playing games to increase the IOPS - such as shorting the seeks by accessing a smaller section of the disk, or making the IO sequential so that requests are serviced from the drive cache. With these tricks you can get the IOPS to exceed 100,000!

Now you can see that it’s difficult to measure and specify IOPS. The number is somewhere between 71 and 100K. For this reason it’s normal to use the worse case, random access pattern value of ~71.

TT