Hi,
On 2021-05-16 18:21:21 -0400, Tom Lane wrote:
> Peter Geoghegan <pg@bowt.ie> writes:
> > On Sun, May 16, 2021 at 1:23 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> And the reason oldestXID contains that is that pg_upgrade applied
> >> pg_resetwal, which does this:
> >> * For the moment, just set oldestXid to a value that will force
> >> * immediate autovacuum-for-wraparound.
>
> > This same pg_resetwal code has probably caused quite a few problems on
> > pg_upgrade'd databases:
> > https://postgr.es/m/20210423234256.hwopuftipdmp3okf@alap3.anarazel.de
>
> Hm, yeah. I'm not sure if transferring the value forward from the
> old cluster is entirely safe, but if it is, that seems like a
> promising route to a fix. (We should still have more sanity checking
> around the GlobalVis code, though.)
Why would it not be safe? Or at least safer than what we're doing right
now? It definitely isn't safe to use a newer value than what the old
cluster used - tables might have older tuples. And an older value than
the old cluster's means that we can either accidentally wrap around
without being protected or that a cluster might shut down to prevent a
wraparound. And after the pg_resetwal we can't assign xids that are
older than set_xid - so it won't become inaccurate?
I think we should remove the heuristic thing from pg_resetwal entirely,
and error out if next-xid is set to something too far away from oldest
xid, unless oldexid is also specified.
Greetings,
Andres Freund
