On Sat, Dec 25, 2010 at 08:37:42PM -0500, Ross Walker wrote: > On Dec 24, 2010, at 1:21 PM, Richard Elling <richard.ell...@gmail.com> wrote: > > > Latency is what matters most. While there is a loose relationship between > > IOPS > > and latency, you really want low latency. For 15krpm drives, the average > > latency > > is 2ms for zero seeks. A decent SSD will beat that by an order of > > magnitude. > > Actually I'd say that latency has a direct relationship to IOPS because it's > the time it takes to perform an IO that determines how many IOs Per Second > that can be performed.
Assuming you have enough synchronous writes and that you can organize them so as to keep the drive at max sustained sequential write bandwidth, then IOPS == bandwidth / logical I/O size. Latency doesn't enter into that formula. Latency does remain though, and will be noticeable to apps doing synchronous operations. Thus 100MB/s, say, sustained sequential write bandwidth with, say, 2KB avg ZIL entries you'd get 51200/s logical, sync write operations. The latency for each such operation would still be 2ms (or whatever it is for the given disk). Since you'd likely have to batch many ZIL writes you'd end up making the latency for some ops longer than 2ms and others shorter, but if you can keep the drive at max sustained seq write bandwidth then the average latency will be 2ms. SSDs are clearly a better choice. BTW, a parallelized tar would greatly help reduce the impact of high latency open()/close() (over NFS) operations... Nico -- _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
