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. > -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- Stick them with the pointy end. ---
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