Re: Behavior for crash recovery when it detects a corrupt WAL record

On Tuesday, October 09, 2012 7:38 PM Heikki Linnakangas wrote:
> On 09.10.2012 16:42, Amit Kapila wrote:
> > I have observed that currently during recovery, while it applies the
> WAL
> > records even if it detects that there is a corrupt record
> >
> > by crc validation, it proceeds.
> >
> > Basically ReadRecord(), returns NULL in such cases which makes the
> behavior
> > same as it has reached end of WAL.
> >
> > After that server get started and user can perform operations
> normally.
> 
> Yeah. We rely on the CRC to detect end of WAL during recovery. If the
> system crashes while the WAL is being flushed to disk, it's normal that
> there's a corrupt (ie. partially written) record at the end of the WAL.
> This is a common technique used by pretty much every system with a
> transaction log / journal.
> 
> The other option would be to perform two fsyncs for every commit; one to
> flush the WAL to disk, and another to update some global pointer to
> point to the end of valid WAL (e.g in pg_control).

Yeah, Can't we check if there is a next valid page, then it can be derived
that
current page has some corruption and not a partial page write problem. 
Though it might not address problem in all scenarios like, with this we
can't identify if there are more valid records on same
Page where we find the CRC problem.

In general, do you think it is a genuine to give such feature to user as we
already have CRC on WAL records, so it is comparatively easy to detect
corruption.

With Regards,
Amit Kapila.