Re: [PATCH 000 of 7] md: Introduction - raid5 reshape mark-2

[Neil Brown]

On Tuesday January 24, lmb@xxxxxxx wrote:
> On 2006-01-24T11:40:47, NeilBrown <neilb@xxxxxxx> wrote:
> 
> > I am expecting that I will ultimately support online conversion of
> > raid5 to raid6 with only one extra device.  This process is not
> > (efficiently) checkpointable and so will be at-your-risk.
> 
> So the best way to go about that, if one wants to keep that option open
> w/o that risk, would be to not create a raid5 in the first place, but a
> raid6 with one disk missing?
> 
> Maybe even have mdadm default to that - as long as just one parity disk
> is missing, no slowdown should happen, right?

Not exactly....

raid6 has rotating parity drives, for both P and Q (the two different
'parity' blocks).
With one missing device, some Ps, some Qs, and some data would be
missing, and you would definitely get a slowdown trying to generate
some of it.

We could define a raid6 layout that didn't rotate Q.  Then you would
be able to do what you suggest.
However it would then be no different from creating a normal raid5 and
supporting online conversion from raid5 to raid6-with-non-rotating-Q.
This conversion doesn't need an reshaping pass, just a recovery of the
now-missing device.

raid6-with-non-rotating-Q would have similar issues to raid4 - one
drive becomes a hot-spot for writes.  I don't know how much of an
issue this really is though.

NeilBrown
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