The problem of simulating random IO
October 23, 2012
Due to recent events, we're now rather interested in being able to measure, characterize, and track our disk IO performance. This, it turns out, presents some problems in the modern world.
Of course the gold standard thing to measure is your actual observed performance in production (and thanks to some work with DTrace, I can now actually do that). However, the problem with production performance is that so many things influence it that it's hard to know what changes in it mean. In particular, if we observe production performance slowing down we don't know if it's because we've got more load, different IO patterns than before, or a genuine problem that we can do something about. So we need to be able to do controlled tests that measure real disk performance, in detail. This means defeating prefetching, which means what we really want to measure is random IO performance.
In theory this is simple enough, given an existing large file; pick a block size, generate or get some high-quality random numbers, seek to those offsets (in blocks), read or write something, and repeat. In practice there is a significant problem with using genuinely random numbers to drive random IO: repeatability (and its closely related cousin, consistency). If I run my test today then run it again in a month and get different results, is that difference because our IO system genuinely changed or because I got different random numbers?
So what I really want is a fixed sequence of non-sequential IO that defeats operating system prefetching (at least). The problem with this in the modern world is that OSes and filesystems are getting disturbingly superintelligent about detecting patterns in your IO. Sequential forward and backwards? That's easy (everyone does at least sequential forwards). Forwards and backwards with a stride? That too. Multiple streams of any of the above, interleaved? There are filesystems that detect it (and I happen to be dealing with one of them). Coming up with a sequence of IO that defeats all of this is what they call an interesting problem.
(And one that I don't have a solution for.)
Which brings me to a small request for filesystem designers: please
provide a way to turn off your superintelligent prefetching for specific
IO. Yes, it's great, but sometimes people want to do real IO right
through to the disks. Having this feature also turn off all caching
(and turn off placing the data in the cache) is optional but probably
appreciated. I suggest that you borrow the Linux
(Providing a filesystem-wide or system-wide flag is not good enough. I don't want to turn off all prefetching on a production filesystem or fileserver so that I can accurately measure disk IO performance; that cure is worse than the disease.)
Written on 23 October 2012.
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