Re: messed up changing chunk size

[Roman Mamedov <roman@rm.pp.ru>]

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On Mon, 19 Jul 2010 14:24:12 +0100
Jools Wills <jools@oxfordinspire.co.uk> 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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