Re: xlog corruption

On Mon, 2012-02-27 at 16:30 -0800, Jameison Martin wrote:
> I'd like to get some clarification around an architectural point about
> recovery. I see that it is normal to see "unexpected pageaddr" errors
> during recovery because of the way Postgres overwrites old log files,
> and thus this is taken to be a normal termination condition, i.e. the
> end of the log (see
> http://doxygen.postgresql.org/xlog_8c.html#a0519e464bfaa79bde3e241e6cff986c7). My question is how does recovery distinguish between the actual end of the log as opposed to a log file corruption (e.g. torn page)?
>
>
> I'd like to be able to distinguish between a corruption in the log vs.
> a normal recovery condition if possible.

If you have a power failure, a torn page in the WAL is expected. Torn
pages in the data pages are fixed up using WAL; but WAL doesn't have
anything under it to prevent/fix torn pages (unless your filesystem
prevents them).

Of course, checksums are used to prevent recovery from attempting to
play a partial or otherwise corrupt WAL record.

What kind of corruption are you trying to detect?

Regards,
Jeff Davis