[dm-devel] BUG in dm/dm-mirror module?

[Phillip Susi]

Jonathan Brassow wrote:
> On a different topic, why are you mirroring the log?  Isn't this 
> somewhat dangerous?
> 
> Let's say that the primary copy of the log dies or goes offline.  You 
> continue on because the log device is still "good".  If your machine 
> crashes and the primary log device is "rediscovered" on bootup, what 
> happens?  The contents of the stale side will be copied - resulting in 
> your log not properly reflecting the state of your mirror device and 
> maybe even leaving inconsistencies.

This is a problem with any mirror, not just one holding a mirror log.

> You might argue that we should update the metadata to exclude the failed 
> primary at the point of failure.  Two things come to mind:
> 1) log I/O will continue until you take action - leaving you open to the 
> scenario above
> 2) it would be simpler to just allocate a new log (since you are 
> changing metadata anyway) and initialize the log as "in-sync" if the 
> mirror is already "in-sync".

Yes, once one drive fails, the metadata on the other drive should 
indicate that the mirror is broken and this is now the most up to date 
copy.