Re: do only critical work during single-user vacuum?

Hi,

On 2022-02-03 13:42:20 -0500, Robert Haas wrote:
> They *do* have a choice. They can continue to operate the system in
> multi-user mode, they can have read access to their data, and they can
> run VACUUM and other non-XID-allocating commands to fix the issue.
> Sure, their application can't run commands that allocate XIDs, but
> it's not going to be able to do that if they go to single-user mode
> either.

I wonder if we shouldn't add some exceptions to the xid allocation
prevention. It makes sense that we don't allow random DML. But it's e.g. often
more realistic to drop / truncate a few tables with unimportant content,
rather than spend the time vacuuming those. We could e.g. allow xid
consumption within VACUUM, TRUNCATE, DROP TABLE / INDEX when run at the top
level for longer than we allow it for anything else.

> But it would still be better than going into single-user mode, which
> provides even less functionality and has basically no advantages of
> any kind.

Indeed. Single user is the worst response to this (and just about anything
else, really). Even just getting into the single user mode takes a while
(shutdown checkpoint). The user interface is completely different (and
awful). The buffer cache is completely cold. The system is slower because
there's no wal writer / checkpointer running. Which basically is a list of
things one absolutely do not wants when confronted with a wraparound
situation.

Greetings,

Andres Freund