On Mon, 19 Jul 2010 14:24:12 +0100 Jools Wills <jools@xxxxxxxxxxxxxxxxxxx> wrote: > >> I looked around and found that chunk size > >> of 512 should work better. > > > > Not true, at least with RAID5/6 a chunk size of 64K performs faster, see > > http://louwrentius.blogspot.com/2010/05/raid-level-and-chunk-size-benchmarks.html > > http://alephnull.com/benchmarks/sata2009/chunksize.html > > There is no mention in this benchmark tests if he modified the stripe > cache. With an increased stripe size a larger stripe cache would be > needed for good performance (at least from my experience) That's true, but increasing the stripe cache helps across all chunk sizes, even with relatively low 128K, see [1]. And I don't think that a large-chunk(512K) configuration will significantly, if at all, outperform a small-chunks(64K) one at any given stripe cache size (set to the same value in both cases). There's also an opinion I heard from a couple of sources, that the stripe size better be chosen so that either one stripe, or a whole stride fits inside the CPU's L2 or L3 cache. Sounds logical, though I am not convinced that this is what causes the performance difference between various stripe sizes. [1] http://peterkieser.com/2009/11/29/raid-mdraid-stripe_cache_size-vs-write-transfer/ -- With respect, Roman
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