Re: staggered stripes

[Russell Coker <russell@coker.com.au>]

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?

> 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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