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Re: changing raid level?

On Wed, Jan 28, 2009 at 08:26:12AM -0600, Greg Oster wrote:
> Manuel Bouyer writes:
> > On Tue, Jan 27, 2009 at 01:23:37PM -0600, Greg Oster wrote:
> > >   [...]
> > > 
> > > For a RAID 1 set, the "parity unit" is simply the second disk, and 
> > > computing that parity requires no further computations.
> > 
> > On a slow CPU the read+read+compare+write is killing the whole system
> > (mostly the "compare" I think) with 90% of the cpu working on this.
> > A simple read+write is much, much better on such a setup (the system
> > is still useable while doing a raid1 reconstruct). It matters
> > even on a fast CPU, when both disks are able to provide data at 180MB/s.
> > The system is still useable but comptationnal tasks (esp. network
> > encryption) is much, much slower.
> What should happen here is that RAIDframe should have a tweakable 
> sysctl knob (or something) to adjust the rebuild rate.  It's on the 
> TODO list, but has never reached any sort of priority level where 
> it's gotten done :(  

Or, perhaps it could measure the reconstruction speed and switch to whichever
 one goes faster, while throttling to ensure X% cpu available for other tasks.
I guess the X would still end up being sysctl knob anyway, but you
 wouldn't need to tweak it b/c you're on a slower (or faster) machine.


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