Re: MD performance options: More CPU’s or more Hz’s?

[jim owens <jowens@hp.com>]

jim owens wrote:
> mark delfman wrote:
>> Thank you Bill... maybe a silly question but doesn’t the fact we can
>> achieve 1.6GBsec on MD RAID0 mean we have drive bandwidth available to
>> MD to use?  To the block device (sas drives in this case) it does not
>> know if the data is been xor’d or just striped, its just blocks of
>> data.  So wouldn’t this mean that we know that MD RAID6 ‘could’
>> achieve up to 1.6GB if the software could send it out quicker?
>>
>> Hence my logic of ‘deal with MD software aspects faster = faster writes’
> 
> Are your tests and assumptions valid... remember that raid6
> will put 1.6GB of data on the drives for each 800MB of data
> from the application in minimum 4 stripe mode (2 data stripes,
> 1 P stripe, 1 Q stripe).
> 
> So the proper comparison of raid6 software overhead is to
> run the same application writing a fixed large amount of data
> to 4 drives at raid0 and the same 4 drives at raid6.
> 
>   overhead = raid0_time * 2 - raid6_time;

UGH... wrote that backwards:

overhead = raid6_time - raid0_time * 2;

> Of course if you add more data stripes, the ratio of
> user data to disk data gets better than 1/2.
> 
> jim
> 
> 

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