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

[jim owens <jowens@hp.com>]

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;

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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