Re: How does btrfs handle bad blocks in raid1?

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

On 2014-01-09 12:31, Chris Murphy wrote:
> 
> On Jan 9, 2014, at 5:52 AM, Austin S Hemmelgarn
> <ahferroin7@gmail.com> wrote:
>> Just a thought, you might consider running btrfs on top of LVM in
>> the interim, it isn't quite as efficient as btrfs by itself, but
>> it does allow N-way mirroring (and the efficiency is much better
>> now that they have switched to RAID1 as the default mirroring
>> backend)
> 
> The problem that in case of mismatches, it's ambiguous which are
> correct.
> 
At the moment that is correct, I've been planning for some time now to
write a patch so that the RAID1 implementation on more than 2 devices
checks what the majority of other devices say about the block, and
then updates all of them with the majority.  Barring a manufacturing
defect or firmware bug, any group of three or more disks is
statistically very unlikely to have a read error at the same place on
each disk until they have accumulated enough bad sectors that they are
totally unusable, so this would allow recovery in a non-degraded RAID1
array in most cases.