Re: BUG in dm/dm-mirror module?

[Phillip Susi <psusi@cfl.rr.com>]

Jonathan Brassow wrote:
> It is a special problem with the mirror log.
> 
> Mirrors will recover themselves and become consistent upon a reboot.  In 
> the case of a mirror that holds a file system, if you lost some of your 
> most recent writes, journaling/fsck will take care of it.  In the case 
> of a mirror that holds another mirror's log, you wind up with a log that 
> does not contain recent data - and could spell coherency issues for the 
> top level mirror.

Having a filesystem that is consistent is still not correct if it is 
older data, at least not when the newer data is available.

> There is no metadata on the other drive, that's part of the problem.  We 
> must discern between metadata that is made by LVM (or other userspace 
> app) and meta-data areas that are known to the device mapper target.  
> Currently, the mirroring target only has the log device - which I 
> contend is insufficient.

LVM needs to update its metadata to indicate that the other drive failed 
and this one now contains more up to date information going forward.