On Sep 12, 2:39 pm, jo...@antarean.org ("J. Roeleveld") wrote:
> On Sunday 12 September 2010 13:32:00 Martijn van Oosterhout wrote:
>
> > On Sun, Sep 12, 2010 at 12:18:10PM +0200, J. Roeleveld wrote:
> > > How can you ensure the snapshot is in a consistent state if the server is
> > > running?
>
> > > If a snapshot is taken between 2 updates in a single transaction, only
> > > half of this transaction is included in the snapshot.
> > > I would never take an LVM (or similar) snapshot of an application that
> > > can't be paused in a way to provide a consistent filesystem.
>
> > That's the trick, the filesystem is always in a consistant state,
> > otherwise how could a database survive a power failure?
>
> This is something you want to try to avoid.
> Recovery situations are not always reliable.
>
> > The trick is WAL, which ensure that changes are logged consistantly and
> > replays them if the database crashes.
>
> > If you take a snapshot the database will simply startup and replay the
> > log as if the machine crashed at the point. All committed transactions
> > appears anything uncommitted vanishes.
>
> Nice in theory.
> Except backups can not be fully trusted if they rely on database recovery
> mechanics as part of the restore process.
>
> How certain can you be that the data you have in your backup will always
> result to being able to recover 100%?
>
> --
> Joost
>
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Just to add that it is standard practice to use database recovery to
bring up a 'Hot Backup'. For example Oracle's default RMAN online
backup if used to restore from will put back the files it backed up
before recovering using the archived and current redo logs (ie the
postgres WAL). It is pretty standard practice to abort 'crash' the
database which is the equivalent to a machine power outage. Recovering
from your 'inconsistent' backup is identical to recovering from you
user generated 'abort'.