Re: [Xen-users] How (not) to destroy a PostgreSQL db in domU on powerfail

[Matthieu Patou <mat+Informatique.xen@xxxxxxxxx>]

On 03/04/2009 05:44 AM, Michael Monnerie wrote:
> On Mittwoch 04 März 2009 Michael Monnerie wrote:
>    
> > On Mittwoch 04 März 2009 Fajar A. Nugraha wrote:
> >      
> > > Is your domU filesystem (/path/root20090113.xvd ?) located on top
> > > of LVM? If yes, then AFAIK LVM does not honor write-barriers.
> > >        
> > Indeed. LVM is bad? I've read LVM is suggested in order to have an
> > easier backup with snapshots. But if the result is that your XEN domU
> > gets destroyed on a power outage - damn LVM.
> >
> > Can someone confirm that LVM does not honor write-barriers?
> >      
> http://lkml.org/lkml/2008/5/16/390
> http://hightechsorcery.com/2008/06/linux-write-barriers-write-caching-
> lvm-and-filesystems
>
> If I understand correctly, turning off disk cache write cache should be
> enough to be save, even when using LVM. Is it really XEN that messed the
> disk, or something else? Could the Areca controller driver do something
> wrong? I'd really love to get a stable and data-secure system, as you
> might understand.
>
> mfg zmi
>    
Mike,
It's quite strange, I am running xen with xfs and lvm since a couple of 
time and I had some server crash (not power cable failure but still).
It's well known that lvm do not honors barriers (as ext3 as well by 
default until last year: http://lkml.org/lkml/2008/5/16/390) so it means 
that you are not completely sure that metadata are written before the 
real data are modified, they can still be in the cache and if it vanish 
(due to power outage on not battery backed controller) then you're on 
your own.
But if you have a battery backed cache then it should be ok, that is to 
my understanding (I can be wrong).
It can be that your battery is dead or not working correctly (or your 
controller not using it ...), the other option is that some metadata 
haven't leaved the os cache (read: the domU has written the information 
to the disk but either dom0 or the hypervisor is mainlining is caching 
data for backend device) and in this case it's normal to face problem.

It would be very interesting to have more information from xfs team.

Matthieu.

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