[CentOS] Re: question about software Raid 1

Nataraj incoming-centos at rjl.com
Sun Sep 21 20:55:45 UTC 2008


On Sun, 2008-09-21 at 12:53 -0700, John R Pierce wrote:
> Kay Diederichs wrote:
> > BTW, there is - even with current kernels - no speed gain in using 
> > RAID1 - see http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing .
> 
> except, thats wrong. I unwrapped a recent kernel source tarball from 
> kernel.org and found...
> 
> static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector)
> {
>         struct mirror *m = get_default_mirror(ms);
> 
>         do {
>                 if (likely(!atomic_read(&m->error_count)))
>                         return m;
> 
>                 if (m-- == ms->mirror)
>                         m += ms->nr_mirrors;
>         } while (m != get_default_mirror(ms));
> 
>         return NULL;
> }
> 
> 
> so it appears its a round robin ...
> _______________________________________________

This makes sense.  I'm pretty sure that tests that I've run in the past
using bonnie++ or iozone showed faster reads with raid1 than with a
single drive.  I would think that if the drives are on seperate
controllers (and depending upon the performance/capacity of the drives
and controllers), there could be notable improvements.

Nataraj




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