Re: Invalid page header

Tom Lane wrote:
>
> You can zap just the failed block by turning on "zero_damaged_pages";
> that will at least allow you to recover the rest of the table.  If you
> want to try harder, you could look at the damaged page with pg_filedump
> (http://sources.redhat.com/rhdb/) or a similar tool and try to intuit
> how to fix it manually.
>

I zapped the damaged block.  It didn't seem to effect the rows in the
table.  My suspicion is that the page only contained deleted rows since
the table had many updates done recently.

> Hmm.  A scribble-on-memory kind of bug could cause this, but in my
> experience it's unusual for coding errors to trash the disk buffers ---
> that's a relatively small part of your address space, and usually a
> memory clobber will crash the backend elsewhere before it hits a disk
> buffer.  (BTW, one reason we force a database restart after a backend
> crash is in hopes of not letting any such clobber make it to disk.  The
> contents of shared disk buffers are simply thrown away in a restart.)
>
> It would probably be worth your while to look at the damaged page with
> pg_filedump before you zap it.  The symptoms of hardware misfeasance and
> software errors are enough different that you can often tell which
> theory to believe by examining the bits.
>

I used pg_filedump on a backup of the database files.  The block looks
like it is mostly zero bytes with a few x02 bytes thrown to just be
confusing.

  - Ian