On Wed, Nov 13, 2002 at 06:37:40AM +1100, Neil Brown wrote:
> On Tuesday November 12, jakob@unthought.net wrote:
> >
> > You might want to consider using huge chunk-sizes when reading, but
> > making sure that writes can be made on "sub-chunks" - so that one could
> > run a RAID-6 with a 128k chunk size, yet have writes performed on 4k
> > chunks. This is important for performance on both read and write, but
> > it is an optimization the current RAID-5 code lacks.
>
> Either I misunderstand your point, or you misunderstand the code.
>
> A 4k write request will cause a 4k write to a data block and a 4k
> write to a parity block, no matter what the chunk size is. (There may
> also be pre-reading, and possibly several 4k writes will share a
> parity block update).
Writes on a 128k chunk array are significantly slower than writes on a
4k chunk array, according to someone else on this list - I wanted to
look into this myself, but now is just a bad time for me (nothing new
on that front).
The benchmark goes:
| some tests on raid5 with 4k and 128k chunk size. The results are as follows:
| Access Spec 4K(MBps) 4K-deg(MBps) 128K(MBps) 128K-deg(MBps)
| 2K Seq Read 23.015089 33.293993 25.415035 32.669278
| 2K Seq Write 27.363041 30.555328 14.185889 16.087862
| 64K Seq Read 22.952559 44.414774 26.02711 44.036993
| 64K Seq Write 25.171833 32.67759 13.97861 15.618126
So down from 27MB/sec to 14MB/sec running 2k-block sequential writes on
a 128k chunk array versus a 4k chunk array (non-degraded).
In degraded mode, the writes degenerate from 30MB/sec to 16MB/sec as the
chunk-size increases.
Something's fishy.
>
> I see no lacking optimisation, but if you do, I would be keen to hear
> a more detailed explanation.
Well if a 4k write really only causes a 4k write to disk, even with a
128k chunk-size array, then something else is happening...
I didn't do the benchmark, and I didn't get to investigate it further
here, so I can't really say much else productive :)
/ Jakob "linux-raid message multiplexer" Østergaard
--
................................................................
: jakob@unthought.net : And I see the elder races, :
:.........................: putrid forms of man :
: Jakob Østergaard : See him rise and claim the earth, :
: OZ9ABN : his downfall is at hand. :
:.........................:............{Konkhra}...............:
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