SATA Performance

ORIGINALLY POSTED 4th August 2007

15,247 views on developerworks

While googling I spotted an interesting topic from Chris, discussing the application of SATA drives in the enterprise. Quite rightly for years most vendors have been telling their customers that SATA is no replacement for SCSI or Fibre-Channel enterprise class disks.

I don’t know enough about the low level data integrity of the drives, I believe that SATA may well suffer more than SCSI or FC with ‘missed-write’ issues (where the drive claims to have written the data but does not) however my gripe with SATA is the performance.

While SATA can cope with reasonable MB per second (MB/s) rates, it simply cannot handle a high number of IO’s per second (IO/s). Meanwhile the size of such ‘cheap’ disks increases, with 1TB not far away from general availability, can you really service 1,000,000 MB of data storage with only around 100 IO/s ?!

Lets look at the facts :

  • 15K RPM SAS drives have surprised me with their ability to handle more 250 IO/s write and over 350 IO/s read ~275 on average
  • 15K RPM FC or SCSI drives can handle around 250 IO/s on average
  • 10K RPM FC or SCSI drives can handle around 180 IO/s on average
  • 7.2K RPM SATA drives can only handle 80-100 IO/s on average

I remember being amazed when we got our first shipment of 9.1GB SSA drives into test back in 1996, most desktops were at the time being shipped with less than 100MB. Magnetic disk media technology really hasn’t moved on very much. So today’s 750GB SATA drives do have more than 85 times that capacity of a ten year old 9.1GB’s drive however only half its performance. Is it any wonder vendors don’t recommend them for the enterprise.

Consolidation, consolidation, consolidation.

Green issues are of political importance to all data-centers today. True virtualization lets you consolidate, meeting its promise to help reduce wasted capacity and cut power bills and your ‘carbon footprint’. Today you can buy a single drive that stores the same as an entire array did last year. But lets not confuse consolidation with data access and performance requirements. There’s still a finite number of heads and platters spinning at a fixed speed inside that drive.

I guess my point here is not only to consider where SATA can be used in your environment, but also that you can only consolidate so far before you start causing performance problems.

Don’t get me wrong, SATA has its uses in the data center. What I have a problem with is user expectations, and maybe more that users are being mislead. I agree that there is no cost-justification in using 15K RPM drives to store a backup, that is hopefully never needed. Just don’t expect your primary systems to be un-affected while you perform your backup to something that can only handle half the IO/s

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