On 2014/05/15 04:38 PM, 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?
I suspect the approach is similar to the following:
sd[abcd][1234....] each configured as LVM PVs
sda[1234....] as an LVM VG
sdb[2345....] as an LVM VG
sdc[3456....] as an LVM VG
sdd[4567....] as an LVM VG
btrfs across all four VGs
^ Um - the above is ignoring "DOS"-style partition limitations
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.
--
__________
Brendan Hide
http://swiftspirit.co.za/
http://www.webafrica.co.za/?AFF1E97
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