On Dec 27, 2010, at 6:06 PM, Edward Ned Harvey wrote: >> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- >> boun...@opensolaris.org] On Behalf Of Nicolas Williams >> >>> 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 > > Ok, what we've hit here is two people using the same word to talk about > different things. Apples to oranges, as it were. Both meanings of "IOPS" > are ok, but context is everything. > > There are drive random IOPS, which is dependent on latency and seek time, > and there is also measured random IOPS above the filesystem layer, which is > not always related to latency or seek time, as described above.
The small, random read model can assume no cache hits. Adding caches makes the model too complicated for simple analysis, and arguably too complicated for modeling at all. For such systems, empirical measurements are possible, but can be overly optimistic. For example, it is relatively trivial to demonstrate 500,000 small, random read IOPS at the application using a file system that caches to RAM. Achieving that performance level for the general case is much less common. -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
