dmeventd doesn't handle failures during mirror resync.

[Nikanth Karthikesan <knikanth@suse.de>]

On Thursday 13 May 2010 04:49:20 Neil Brown wrote:
> Thanks for the reply,
> 
> On Wed, 05 May 2010 10:08:09 +0200
> 
> Petr Rockai <prockai@redhat.com> wrote:
> > Neil Brown <neilb@suse.de> writes:
> > > I was surprised to discover that while a normal write error is
> > > handled properly - dmeventd runs 'lvconvert' to fix the array up,
> > > this does not happen in response to a write error while syncing
> > > the array.
> > >
> > > If I arrange for the new device to die, then
> > >           lvconvert --repair --use-policies
> > >
> > > will fix it up as I would expect, but dmeventd never asks it to do
> > > this.
> > >
> > > This seems to be a deliberate decision:  in _process_status_code
> > > in dmeventd_mirror.c, a status of 'F' will cause lvconvert to be
> > > run while 'S' and 'R' (sync and read errors) will not.
> > >
> > > Is there a reason for this?
> >
> > I think the rationale is that:
> >
> > For read errors, we should *not* strip the mirror leg, since we want to
> > keep as much redundancy as possible in this scenario. The failure should
> > be logged, but I think that's it.
> 
> In general I would agree, though there are cases where you would want to
> strip a leg based only on read errors.  If a drive is failing in a way that
> all read attempts cause multiple retries and timeouts, then having that
> device remain in the array can kill performance.
> 

On most workloads, on such cases, some write failure would occur sooner, which 
should fix this?

> It seems therefore that this should be a policy based decision, possibly
> involving extra testing, and the place for this would seem to be in
>  lvconvert.
> 
> Hence the thrust of my argument, which is that dmeventd should always run
> lvconvert on any event that is at all suspect, and lvconvert should make
>  the correct decision, which may be to leave the array alone.
> 
> Maybe lvconvert doesn't currently always make the 'correct' decision so
> changing dmeventd might be premature, but I would like to know if you agree
> that this is the best long-term strategy.
> 

Letting lvconvert decide based on policy, looks like a good idea to me.

> > For sync, I am not sure. It may be that the reason for this is that sync
> > is usually related to manual action and dmeventd intervention may be
> > unexpected and unwanted in this case. But that case could be argued.
> 
> If you are using dmeventd, then a write failure could easily cause
>  lvconvert to kick off a sync on a new device, so it could easily not be a
>  manual action.
> 
> 
> The main problem I see with the current arrangement is that if a device
>  goes missing during a sync and we don't 'lvconvert repair', then the next
>  restart will not succeed with "--partial" and that would cause problems
>  rebooting.
> 

I guess, s/with/without.

May be --partial should be used by default, till this is solved? Or repair 
should be done before shutdown.

Thanks
Nikanth