[lvm-devel] [RFC][PATCH 0/5] dmeventd device filtering

[malahal at us.ibm.com]

Petr Rockai [prockai at redhat.com] wrote:
> Hmm. Does this introduce some race conditions? When a bad sequence of metadata
> edits and failures happens, could this lead to bad behaviour? I have skimmed
> the patches and I think following may happen:
> 
> - vgextend a volume group (adding say /dev/sde)
> - metadata is written and committed
> - dmeventd notices a failure, but its device list is out of date 
> - lvconvert does its job, but when writing metadata, it marks the /dev/sde PV
>   as missing, since it can't find it
> - dmeventd triggers vgreduce, which removes /dev/sde from the volume group
> 
> It is not a fatal problem, but definitely surprising. Maybe we could fix it,
> although I'm not entirely sure how.
> 
> Also, I'm a little worried that this is something that may rather easily go out
> of sync -- keeping a cached copy of data like this around is always
> dangerous. Fortunately, the worst that should happen is that an automatic
> recovery fails or that empty PVs are removed from the volume group (like above)
> -- it shouldn't be possible to trick dmeventd into clobbering any data this
> way. Either way -- I am not sure it is a showstopper, but it's definitely not
> very nice. Thoughts?
> 
> Yours,
>    Petr.

How about vgreduce only not scanning the failed devices. It will scan
/dev/sde in the above case. Multiple device failures at the same (not
uncommon) is going to be a problem though. :-(