Re: Random IO with md raid

[Goswin von Brederlow <goswin-v-b@web.de>]

Matthieu Patou <mat@matws.net> writes:

> On 05/12/2009 17:20, Asdo wrote:
>> Matthieu Patou wrote:
>>> * 1 raid 1 volume of 2 1TB hard drive
>> Are these drives also on the 3ware?
> I suppose that you are speaking of the 2 single drives. If so yes they
> are also connected to the 3ware controller, they are not exported as
> jbod but as seperate hard drives.
> Are they exported as jbod?
>>> * 2 single volume of 1 TB hard drive each
>>> ...
>>> Does anyone has an any idea ?
>>
>> Try anticipatory, deadline and noop schedulers on disks (not CFQ)
>> Try setting readahead to exactly 4K, or set it to lowest possible value
> I tried all the different scheduler both with 4K and with the default
> value and I still have raid1 software that do 1/2 or 1/3 of the raid1
> hardware for random write I/O.
>> (I'm not sure what is best), since this is random access...
>> Increase stripe_cache_size as high as you can, p
> So for raid1 there is no stripe_cache_size so I didn't set it ...
>> Let us/me know the results afterwards...
>
> I also made a simple tests with the CFQ scheduler without any other
> tunning but turning of the controller cache and at this moment the
> results of the RAID1 hardware and the RAID1 software are very close.
> Looks like linux is not able to use the onboard cache of controller as
> efficiently as the 3ware controller it self at least for random IO.
>
> Any other ideas ?
>
> Matthieu

Software raid1 will write twice as much data. That means twice as much
data goes over the system bus and into the controler cache.
Effectively you have halve the cache size. Maybe that is all you see.

MfG
        Goswin