Re: RAIDframe parity rebuild (was: RAIDframe performance vs. stripe size: Test Results)

[Greg Oster <oster%cs.usask.ca@localhost>]

On Tue, 12 Jun 2012 18:34:52 +0200
Edgar Fuß <ef%math.uni-bonn.de@localhost> wrote:

> > So a parity rebuild does so by reading all the data and the
> > exiting parity, computing the new parity, and then comparing the
> > existing parity with the new parity.  If they match, it's on to the
> > next stripe.  If they differ, the new parity is written out.
> Oops.
> What's the point of not simply writing out the computed parity?

Writes are typically slower, so not having to do them means the rebuild
goes faster...

> > What we don't know here is what the state of the array was when you
> > started the rebuild.  That is, was the parity 'mostly correct'
> > beforehand? (i.e. saving having to do a lot of the writes).
> I don't know either. If the discs contained a clean RAID set with a
> different SPSU, was the parity correct?

For a basic RAID 5 set yes, it would be.  The exception might be at the
very end of the RAID set where a smaller stripe size might allow you to
use a bit more of each component.

Later...

Greg Oster