Re: Hot standby, prepared xacts, locks

Simon Riggs wrote:
> On Wed, 2009-10-21 at 23:02 +0300, Heikki Linnakangas wrote:
>> Hmm, dunno about that, but there is one problem with the "grant to dummy
>> proc, then release in startup process" approach. What if there isn't
>> enough shared memory available to re-acquire the lock for the dummy
>> proc? It would be rather unfortunate to throw an error and shut down the
>> standby, instead of promoting it to a new master.
>
> Any error would be unfortunate at that point. That particular error
> seems unlikely, since we are only talking about AccessExclusiveLocks. If
> the server has a problem with that many locks, then it is severely in
> danger from prepared transactions in the general case, since such errors
> could be also be thrown by the current code in mildly different
> circumstances.

Note that read-only backends also occupy lock space when they run
queries, taking AccessShareLocks.

> Do you see any alternative approaches to the one taken?

Making some effort to transfer locks instead of acquiring+releasing
would eliminate the need for having extra lock space available when
switching from hot standby mode to normal operation.

>> In fact, what happens if you ran out of shared memory when replaying a
>> relation_redo_lock record? Panic?
>
> An ERROR in the startup process will cause it to upgrade to FATAL,
> AFAIK. That means the server will do a crash shutdown, AIUI. That is the
> equivalent of a PANIC, I guess. How else could it behave?

It could wait for backends to release locks. If there isn't any, then
you don't have much choice.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com