Re: (was: Re: md: raid5 vs raid10 (f2,n2,o2) benchmarks [w/10 raptors])

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Thu, Apr 03, 2008 at 12:02:46PM +0200, Keld Jørn Simonsen wrote:
> On Wed, Apr 02, 2008 at 01:49:44PM -0400, Justin Piszcz wrote:
> > 
> > 
> > On Wed, 2 Apr 2008, Justin Piszcz wrote:
> > 
> > >
> > >
> > >On Wed, 2 Apr 2008, Conway S. Smith wrote:
> 
> I have referenced both of your benchmaks in the wiki on performance. So
> now I just hope that your URLs will live forever. I also took down some
> of your recomendations there. 
> 
> I note that raid10,f2 has a much higher cpu load than raid10,n2 or
> raid10,o2. How come? it is 31-38 % for f2, where n2 and o2 is around 15 %.

I found a reason for this, it seems that CPU usage and IO speed are very
related, so because the raid10,f2 has about double the IO performance
for sequential reading, it also has about double the cpu use.

Justin's benchmark is on http://home.comcast.net/~jpiszcz/20080329-raid/

Another of Justin's benchmarks also reveals the relation between
IO rate and CPU use:
http://home.comcast.net/~jpiszcz/raid/20080528/raid-levels.html

Why does IO use that much CPU? Is it mostly moving around the data from
the kernel to the user space? Does it matter here whether one is running
a 32 bit or a 64 bit system?

It seems like the RAM bus can be a bottleneck. I read that DDR-400
can have a peak performance of 1600 MB/s. If this is halved on 32 bit OS
then this is 800 MB/s. And you need to both read and write when you move
things around. So that is 400 MB/s... And you need to still be able to 
read in from the disk controller at 330 MB/s. For 64 bit systems this is
a max around 400 MB/s - given that there is a flow from the disk
controller to system disk buffers, then from kernel buffers to user
buffers and then from user buffers to some processing. Or am I wrong
here?

Best regards
Keld
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