| From: | Greg Stark <stark(at)mit(dot)edu> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Jim Nasby <jim(at)nasby(dot)net>, Jeff Davis <pgsql(at)j-davis(dot)com>, Florian Pflug <fgp(at)phlo(dot)org>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: corrupt pages detected by enabling checksums |
| Date: | 2013-05-10 12:39:10 |
| Message-ID: | CAM-w4HMzQ-88e4pYNcAuwrW_E64J_a1HbyxY9q841qwkS5GopA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, May 10, 2013 at 7:44 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> Having one corrupt record followed by a valid record is not an
>> abnormal situation. It could easily be the correct end of WAL.
>
> I disagree, that *is* an abnormal situation and would not be the
> "correct end-of-WAL".
>
> Each WAL record contains a "prev" pointer to the last WAL record. So
> for the next record to be valid the prev pointer would need to be
> exactly correct.
Well then you're wrong. The OS makes no guarantee that blocks are
written out in order. When the system crashes any random subset of the
blocks written but not synced might have been written to disk and
others not. There could be megabytes of correct WAL written with just
one block in the middle of the first record not written. If no xlog
sync had occurred (or one was in progress but not completed) then
that's the correct end of WAL.
--
greg
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