Re: Checksum errors in pg_stat_database

On Mon, Dec 12, 2022 at 12:40 AM Michael Paquier <michael(at)paquier(dot)xyz>
wrote:

> On Sun, Dec 11, 2022 at 09:18:42PM +0100, Magnus Hagander wrote:
> > It would be less of a concern yes, but I think it still would be a
> concern.
> > If you have a large amount of corruption you could quickly get to
> millions
> > of rows to keep track of which would definitely be a problem in shared
> > memory as well, wouldn't it?
>
> Yes. I have discussed this item with Bertrand off-list and I share
> the same concern. This would lead to an lot of extra workload on a
> large seqscan for a corrupted relation when the stats are written
> (shutdown delay) while bloating shared memory with potentially
> millions of items even if variable lists are handled through a dshash
> and DSM.
>
> > But perhaps we could keep a list of "the last 100 checksum failures" or
> > something like that?
>
> Applying a threshold is one solution. Now, a second thing I have seen
> in the past is that some disk partitions were busted but not others,
> and the current database-level counters are not enough to make a
> difference when it comes to grab patterns in this area. A list of the
> last N failures may be able to show some pattern, but that would be
> like analyzing things with a lot of noise without a clear conclusion.

Anyway, the workload caused by the threshold number had better be
> measured before being decided (large set of relation files with a full
> range of blocks corrupted, much better if these are in the OS cache
> when scanned), which does not change the need of a benchmark.
>
> What about just adding a counter tracking the number of checksum
> failures for relfilenodes in a new structure related to them (note
> that I did not write PgStat_StatTabEntry)?
>
> If we do that, it is then possible to cross-check the failures with
> tablespaces, which would point to disk areas that are more sensitive
> to corruption.
>

If that's the concern, then perhaps the level we should be tracking things
on is tablespace? We don't have any stats per tablespace today I believe,
but that doesn't mean we couldn't create that.

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>