On Tue, Mar 2, 2021 at 7:52 AM David Rowley <dgrowle...@gmail.com> wrote: > > On Tue, 2 Mar 2021 at 04:32, Hannu Krosing <han...@google.com> wrote: > > > > It looks like we are unnecessarily instructing our usiers to vacuum their > > databases in single-user mode when just vacuuming would be enough. > > > > We should fix the error message to be less misleading. > > It would be good to change the message as it's pretty outdated. Back > in 8ad3965a1 (2005) when the message was added, SELECT and VACUUM > would have called GetNewTransactionId(). That's no longer the case. > We only do that when we actually need an XID. > > However, I wonder if it's worth going a few steps further to try and > reduce the chances of that message being seen in the first place. > Maybe it's worth considering ditching any (auto)vacuum cost limits for > any table which is within X transaction from wrapping around. > Likewise for "VACUUM;" when the database's datfrozenxid is getting > dangerously high. > > Such "emergency" vacuums could be noted in the auto-vacuum log and > NOTICEd or WARNING sent to the user during manual VACUUMs. Maybe the > value of X could be xidStopLimit minus a hundred million or so. > > I have seen it happen that an instance has a vacuum_cost_limit set and > someone did start the database in single-user mode, per the advice of > the error message only to find that the VACUUM took a very long time > due to the restrictive cost limit. I struggle to imagine why anyone > wouldn't want the vacuum to run as quickly as possible in that > situation.
Multiple instances running on the same hardware and only one of them being in trouble? But it would probably be worthwhile throwing a WARNING if vacuum is run with cost delay enabled in single user mode -- so that the user is at least aware of the choice (and can cancel and try again). Maybe even a warning directly when starting up a single user session, to let them know? -- Magnus Hagander Me: https://www.hagander.net/ Work: https://www.redpill-linpro.com/
