From: | Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr>, Bruce Momjian <bruce(at)momjian(dot)us>, Magnus Hagander <magnus(at)hagander(dot)net>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Peter Eisentraut <peter_e(at)gmx(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: pg_basebackup for streaming base backups |
Date: | 2011-01-20 19:10:13 |
Message-ID: | m2hbd34ciy.fsf@2ndQuadrant.fr |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)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
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