Re: locate DB corruption

On Fri, Aug 31, 2018 at 5:19 PM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 08/31/2018 08:51 AM, Dave Peticolas wrote:
> > On Fri, Aug 31, 2018 at 8:14 AM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com
> > <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:
> >
> > On 08/31/2018 08:02 AM, Dave Peticolas wrote:
> > > Hello, I'm running into the following error running a large query
> > on a
> > > database restored from WAL replay:
> > >
> > > could not access status of transaction 330569126
> > > DETAIL: Could not open file "pg_clog/0C68": No such file or
> directory
> >
> >
> > Postgres version?
> >
> >
> > Right! Sorry, that original email didn't have a lot of info. This is
> > 9.6.9 restoring a backup from 9.6.8.
> >
> > Where is the replay coming from?
> >
> >
> > From a snapshot and WAL files stored in Amazon S3.
>
> Seems the process is not creating a consistent backup.
>

This time, yes. This setup has been working for almost two years with
probably hundreds of restores in that time. But nothing's perfect I guess :)

> How are they being generated?
>

The snapshots are sent to S3 via a tar process after calling the start
backup function. I am following the postgres docs here. The WAL files are
just copied to S3.

>
> > Are you sure you are not working across versions?
> >
> >
> > I am sure, they are all 9.6.
> >
> > If not do pg_clog/ and 0C68 actually exist?
> >
> >
> > pg_clog definitely exists, but 0C68 does not. I think I have
> > subsequently found the precise row in the specific table that seems to
> > be the problem. Specifically I can select * from TABLE where id = BADID
> > - 1 or id = BADID + 1 and the query returns. I get the error if I select
> > the row with the bad ID.
> >
> > Now what I'm not sure of is how to fix.
>
> One thing I can think of is to rebuild from a later version of your S3
> data and see if it has all the necessary files.
>

Yes, I think that's a good idea, I'm trying that.

> There is also pg_resetxlog:
>
> https://www.postgresql.org/docs/9.6/static/app-pgresetxlog.html
>
> I have not used it, so I can not offer much in the way of tips. Just
> from reading the docs I would suggest stopping the server and then
> creating a backup of $PG_DATA(if possible) before using pg_resetxlog.
>

Thanks, I didn't know about that. The primary DB seems OK so hopefully it
won't be needed.