On Dec 25, 2010, at 5:37 PM, 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.
That is only true when there is one queue and one server (in the queueing context). This is not the case where there are multiple concurrent I/O that can be completed out of order by multiple servers working in parallel (eg. disk subsystems). For an extreme example, the Sun Storage F5100 Array specifications show 1.6 million random read IOPS @ 4KB. But instead of an average latency of 625 nanoseconds, it shows an average latency of 0.378 milliseconds. The analogy we've used in parallel computing for many years is "nine women cannot make a baby in one month." > Ever notice how storage vendors list their max IOPS in 512 byte sequential IO > workloads and sustained throughput in 1MB+ sequential IO workloads. Only SSD > makers list their random IOPS workload numbers and their 4K IO workload > numbers. The vendor will present the number that makes them look best, often without regard for practical application... the "curse of marketing" :-) -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
