Re: staggered stripes

[Hugo Mills <hugo@carfax.org.uk>]

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On Fri, May 16, 2014 at 12:38:04AM +1000, Russell Coker wrote:
> On Thu, 15 May 2014 09:31:42 Duncan wrote:
> > > Does the BTRFS RAID functionality do such staggered stripes?  If not
> > > could it be added?
> > 
> > AFAIK nothing like that yet, but it's reasonably likely to be implemented
> > later.  N-way-mirroring is roadmapped for next up after raid56
> > completion, however.
> 
> It's RAID-5/6 when we really need such staggering.  It's a reasonably common 
> configuration choice to use two different brands of disk for a RAID-1 array.  
> As the correlation between parts of the disks with errors only applied to 
> disks of the same make and model (and this is expected due to 
> firmware/manufacturing issues) the people who care about such things on RAID-1 
> have probably already dealt with the issue.
> 
> > You do mention the partition alternative, but not as I'd do it for such a
> > case.  Instead of doing a different sized buffer partition (or using the
> > mkfs.btrfs option to start at some offset into the device) on each
> > device, I'd simply do multiple partitions and reorder them on each
> > device.
> 
> If there are multiple partitions on a device then that will probably make 
> performance suck.  Also does BTRFS even allow special treatment of them or 
> will it put two copies from a RAID-10 on the same disk?

   It will do. However, we should be able to fix that with the new
allocator, if I ever get it finished...

   Hugo.

> > Tho N-way-mirroring would sure help here too, since if a given
> > area around the same address is assumed to be weak on each device, I'd
> > sure like greater than the current 2-way-mirroring, even if if I had a
> > different filesystem/partition at that spot on each one, since with only
> > two-way-mirroring if one copy is assumed to be weak, guess what, you're
> > down to only one reasonably reliable copy now, and that's not a good spot
> > to be in if that one copy happens to be hit by a cosmic ray or otherwise
> > fail checksum, without another reliable copy to fix it since that other
> > copy is in the weak area already.
> > 
> > Another alternative would be using something like mdraid's raid10 "far"
> > layout, with btrfs on top of that...
> 
> In the "copies= option" thread Brendan Hide stated that this sort of thing is 
> planned.
> 

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