paper about ZCAV
This paper by Rodney Van Meter about ZCAV (Zoned Constant Angular Velocity) in hard drives is very interesting. It predates my work by about four years and includes some interesting methods of collecting data that I never considered.
One interesting thing is that apparently on some SCSI drives you can get the drive to tell you where the zones are. If I get enough spare time I would like to repeat such tests and see how the data returned by disks compares to benchmark results.
It's also interesting to note that Rodney's paper shows a fairly linear drop of performance on higher sector numbers (while he notes that it would be expected to fall off more quickly at higher sector numbers). One of my recent tests with a 300G disk showed the greater than linear performance drop (see my ZCAV results page for more details). It might require modern large disks to show this performance characteristic.
I also found it very interesting to see that a modified version of Bonnie was used for some of the tests and that it gave consistent results! I assumed that any filesystem based tests of ZCAV performance would introduce unreasonable amounts of variance into my tests and instead wrote my ZCAV test program to directly read the disk and measure performance.
It's times like this that I wish for a "groundhog day" so that I could spend a year doing nothing but reading technical papers.
1 comment:
Hiya,
Thanks for the plug for my old USENIX paper :-). Tomorrow I lecture my architecture students on I/O systems, and somehow I doubt I'll get past talking about disk drives in an hour and a half -- they're just too fascinating. I owe it to them to talk about buses, device drivers, RAID, and some other things, too, but that may have to wait...
I ran across your ZCAV notes a while back, too. Your work is very thorough, I'm really impressed.
I think to see a real deviation from linear in the performance, you have to have a high outer:inner diameter ratio, but I would expect that to be low in laptop drives. Hmm.
It's true that you can see the zoning effects even through the file system, which was the point of my paper, but I started with a large, mostly empty partition. You can see weird effects in SunOS's allocation policy as the cylinder groups start to fill.
I think I still have some of the source code for those tools around somewhere. Keith Smith and John DiMarco were kind enough to share, then I modified them somewhat to get what I needed.
--Rod
Post a Comment