Re: RAID5 -> RAID6

[Max Waterman <davidmaxwaterman@fastmail.co.uk>]

NeilBrown wrote:
> Wait 3 months :-)
>   

Sounds good. I'm in no particular hurry. Increasing capacity would be 
nice, but I'm not sure I want to do that since I only have a 1TB drive 
for backup...as such, the slow version of 1a/ sounds reasonable - I have 
a spare 80GB drive in the same machine that I could make use of to make 
it not-so-dangerous.

I guess I might consider a grow too - perhaps I'll have another drive by 
then so my backup can be bigger.

Thanks for the advice...I'll keep an eye out for the new support.

Max.

> 2.6.30 should contains support for this sort of conversion.  It is
> already written (mostly) but still needs some testing.
>
>
> Your options would then include:
>  1/  convert that raid5 to a raid6 of the same size but with one
>      extra device.  This device would store all the 'Q' blocks so
>      it could become a write bottle neck
>  1a/ as above, but then restripe the array so that the Q block is
>      rotated among the drives.  This process is either dangerous - in
>      that a crash would kill your data, or slow - in that all the data
>      would need to be copied elsewhere in chunks while the corresponding
>      chunk of the array was restriped.
>  2/  convert to raid6 and grow at the same time.  i.e. add both spares
>      using one of them to support the conversion to raid6 and the
>      other to increase the space.  You could then arrange to restripe
>      an grow at the same time which is faster/safer than striping in-place.
>  3/  Possibly you could restripe-and-grow, then restripe-and-shrink
>      so you end up with a 7 device RAID6 with properly rotating parity,
>      but don't go through the slow/dangerous restripe-in-place.
>      I'll need to do some experiments to see if that would actually
>      be faster