Re: RAID 5 doesn't scale

[Peter Landmann <sfrazt@googlemail.com>]

> 
> On 04/03/2013 03:18 PM, Stan Hoeppner wrote:
> 
> > You didn't mention your stripe_cache_size value.  It'll make a lot of
> > difference.  Make sure it's at least 4096.  The default is 256.
> 
> I'm not getting it - why would stripe cache size matter in a random
> read/write test? If the disks are large enough and the pattern is really
> random, the cache should hardly ever be hit (s_c_z = 4096 =^ 16MB cache
> per disk, that's 0.01% of disk size for a 160GB SSD).
> 
> I read that Peter confirmed the influence of stripe_cache_size, but I'd
> like to understand why it matters in this case.
> 
> Martin


I'm very sorry but now i can't confirm anymore that stripe_cache_size helps.

My test was to short. With every minute the IOPS decrease. So 
stripe_cache_size does only help for very short tests. 

I will provide details in another post.

Peter