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Re: Software RAID...

On Wed, 1 Aug 2001 08:31, Roger Abrahamsson wrote:
> > LVM is the correct solution to this problem.  You can run LVM over
> > multiple RAID-5 and RAID-1 arrays.  Then you have RAID for reliability
> > and LVM to allow easy growth of storage.
> >
> > I'm still not sure that LVM is ready for serious use though...
> Hmm, LVM could solve it in a fashion.. However I would have to build one
> new raid5 array each time I uppgraded it with that solution. It would have
> been nice to be able to grow it with one disk at a time. I just thought it
> would be possible to do it, since it's all done in software in any
> case. As for serious use, this is still an "el cheapo" solution. It's okay
> for some downtime, as long as the data stays intact.

The way RAID-5 works with 3 disks is to put blocks in the following order (X 
is an XOR parity block):

0 1 X
X 2 3
4 X 5
6 7 X

With 4 disks it will be the following:
0 1 2 X
X 3 4 5
6 X 7 8

So adding an extra disk would involve moving every block of data (apart from 
the first two).  Now what happens if you experience a crash or power failure 
while doing such a re-build?

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