Re: io performance...

[Ian Soboroff]

Max Waterman <davidmaxwaterman+kernel@xxxxxxxxxxxxxx> writes:

> Phillip Susi wrote:
>> Right, the kernel does not know how many disks are in the array, so
>> it can't automatically increase the readahead.  I'd say increasing
>> the readahead manually should solve your throughput issues.
>
> Any guesses for a good number?
>
> We're in RAID10 (2+2) at the moment on 2.6.8-smp. These are the block
> numbers I'm getting using bonnie++ :
>
>[...]
> We're still wondering why rd performance is so low - seems to be the
> same as a single drive. RAID10 should be the same performance as RAID0
> over two drives, shouldn't it?

I think bonnie++ measures accesses to many small files (INN-like
simulation) and database accesses.  These are random accesses, which
is the worst access pattern for RAID.  Seek time in a RAID equals the
longest of all the drives in the RAID, rather than the average.  So
bonnie++ is domninated by your seek time.

Ian


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