Re: Why do I need 4 disks for a raid6?

[Robin Hill <robin@robinhill.me.uk>]

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On Wed Mar 18, 2009 at 03:08:56PM +0100, Goswin von Brederlow wrote:

> Andre Noll <maan@systemlinux.org> writes:
> 
> > On 08:34, Ruslan Sivak wrote:
> >
> >> I would guess the reason is that it doesn't make sense.  As mentioned, if
> >> you are going to create a 3 disk raid 6, it's essentially a raid1 over 3
> >> disks, at which point you are better off with the raid-1.  I don't think
> >> there's a raid controller that would let you set something like this up, I
> >> don't see why the softraid should.
> >
> > Well, Goswin mentioned some pretty good reasons I think.
> >
> 
> Raid6 is the only level that requires such "sanity".
> 
> Here is another scenario: Say you have a 2 disk raid1 and now want to
> switch to 5 disk raid6. No problem, add 3 new disks, set up 1+2 disk
> raid6, pvmove the data (don't you love LVM?), stop the raid1, grow the
> raid6 to 5 disks.
> 
> It is clear that 1+2 disk raid6 only makes sense as a transitory step
> but one that is usefull.
> 
You could do this via the new RAID5-RAID6 migration. Either:
    - recreate the two disk RAID1 as RAID5 (assume clean)
    - add 2 disks and grow the array
    - add the fifth disk and convert to RAID6
or
    - create a new 3-disk RAID5 array
    - copy/pvmove the data
    - add a fourth disk and grow the array
    - add the fifth disk and convert to RAID6
or even (not 100% sure this'll work until the RAID6 restriping work is
done):
    - create a new 3-disk RAID5 array
    - copy/pvmove the data
    - add a fourth disk and convert to RAID6
    - add the fifth disk and grow the array

So the 3-disk RAID6 isn't essential, though if there's no particular
reason to prohibit it then it might as well be allowed as an option.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
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