Re: pg_basebackup for streaming base backups

Robert Haas <robertmhaas@gmail.com> writes:
> I think that the basic problem with wal_level is that to increase it
> you need to somehow ensure that all the backends have the new setting,
> and then checkpoint.  Right now, the backends get the value through
> the GUC machinery, and so there's no particular bound on how long it
> could take for them to pick up the new value.  I think if we could
> find some way of making sure that the backends got the new value in a
> reasonably timely fashion, we'd be pretty close to being able to do
> this.  But it's hard to see how to do that.

Well, you just said when to force the "reload" to take effect: at
checkpoint time.  IIRC we already multiplex SIGUSR1, is that possible to
add that behavior here?  And signal every backend at checkpoint time
when wal_level has changed?

> I had some vague idea of creating a mechanism for broadcasting
> critical parameter changes.  You'd make a structure in shared memory
> containing the "canonical" values of wal_level and all other critical
> variables, and the structure would also contain a 64-bit counter.
> Whenever you want to make a parameter change, you lock the structure,
> make your change, bump the counter, and release the lock.  Then,
> there's a second structure, also in shared memory, where backends
> report the value that the counter had the last time they updated their
> local copies of the structure from the shared structure.  You can
> watch that to find out when everyone's guaranteed to have the new
> value.  If someone doesn't respond quickly enough, you could send them
> a signal to get them moving.  What would really be ideal is if you
> could actually make this safe enough that the interrupt service
> routine could do all the work, rather than just setting a flag.  Or
> maybe CHECK_FOR_INTERRUPTS().  If you can't make it safe enough to put
> it in someplace pretty low-level like that, the whole idea might fall
> apart, because it wouldn't be useful to have a way of doing this that
> mostly works except sometimes it just sits there and hangs for a
> really long time.
>
> All pie in the sky at this point...

Unless we manage to simplify enough the idea to have wal_level SIGHUP.

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support