On Thu, Dec 23, 2010 at 11:25:43AM +0100, Stephan Budach wrote: > as I have learned from the discussion about which SSD to use as ZIL > drives, I stumbled across this article, that discusses short > stroking for increasing IOPs on SAS and SATA drives:
There was a thread on this a while back. I forget when or the subject. But yes, you could even use 7200 rpm drives to make a fast ZIL device. The trick is the on-disk format, and the pseudo-device driver that you would have to layer on top of the actual device(s) to get such performance. The key is that sustained sequential I/O rates for disks can be quite large, so if you organize the disk in a log form and use the outer tracks only, then you can get pretend to have awesome write IOPS for a disk (but NOT read IOPs). But it's not necessarily as cheap as you might think. You'd be making very inefficient use of an expensive disk (in the case of an SAS 15k rpm disk), or disks, and if plural then you are also using more ports (oops). Disks used this way probably also consume more power than SSDs (OK, this part of my analysis if very iffy), and you still need to do something about ensuring syncs to disk on power failure (such as just disabling the cache on the disk, but this would lower performance, increasing the cost). When you factor all the costs in I suspect you'll find that SSDs are priced reasonably well. That's not to say that one could not put together a disk-based log device that could eat SSDs' lunch, but SSD prices would then just come down to match that -- and you can expect SSD prices to come down anyways, as with any new technologies. I don't mean to discourage you, just to point out that there's plenty of work to do to make "short-stroked disks as ZILs" a workable reality, while the economics of doing that work versus waiting for SSD prices to come down don't seem appealing. Caveat emptor: my analysis is off-the-cuff; I could be wrong. Nico -- _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
