Re: Re: Backup and Recovery

On Wed, Jul 04, 2001 at 10:37:57AM +0200, Zeugswetter Andreas SB wrote:
>
> > I imagine a daemon extracting redo log entries from WAL segments,
> > asynchronously. Mixing redo log entries into the WAL allows the WAL
> > to be the only synchronous disk writer in the system, a Good Thing.
>
> This comes up periodically now. WAL currently already has all the info
> that would be needed for redo (it actually has to).

The WAL has the information needed to take a binary table image
from the checkpoint state to the last committed transaction.
IIUC, it is meaningless in relation to a pg_dump image.

> All that is missing is a program, that can take a consistent physical
> snapshot (as it was after a particular checkpoint) and would replay
> the WAL after a restore of such a snapshot. This replay after a
> consistent snapshot is probably as simple as making the WAL files
> available to the standard startup rollforward (redo) mechanism, that
> is already implemented.

How would you take a physical snapshot without interrupting database
operation? Is a physical/binary snapshot a desirable backup format?
People seem to want to be able to restore from ASCII dumps.

Also, isn't the WAL format rather bulky to archive hours and hours of?
I would expect high-level transaction redo records to be much more compact;
mixed into the WAL, such records shouldn't make the WAL grow much faster.

Nathan Myers
ncm(at)zembu(dot)com