From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Magnus Hagander <magnus(at)hagander(dot)net> |
Cc: | Andreas Kling <andreas(dot)kling(at)acgnystrom(dot)se>, pgsql-bugs(at)postgresql(dot)org |
Subject: | Re: [PATCH] Don't bail with legitimate -N/-B options |
Date: | 2008-02-16 22:23:44 |
Message-ID: | 10846.1203200624@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
Magnus Hagander <magnus(at)hagander(dot)net> writes:
> Those are not comments on the actual patch, of course. For that one, it
> looks to me like it's the wrong fix. I don't think we should be adding
> to shared buffers like that - if somebody asked for a specific value
> they should get that.
Agreed, this is really inappropriate.
The whole test probably needs to be rethought. I think the idea
was to ensure that you couldn't run out of pinnable buffers if all
backends were concurrently doing UPDATEs that touched two pages. But
it's an underestimate --- for instance, if the UPDATE is doing an
indexscan then there's likely an index page pinned somewhere as well.
Worse, if you're joining N tables then there's likely N or N*2 pages
pinned at any instant.
Personally what I think we should do is intentionally break the current
Gentoo packaging --- we already unintentionally broke it, but changing
the code that those ridiculous parameter values are accepted again is
NOT the appropriate next step. I'd be for raising the minimum -B to a
couple hundred. I'm not sure if we should continue to enforce any
minimum -B-to-N ratio, but if we do, let's make sure that it's measured
without counting the autovac processes, so as to keep the error message
simple.
I think at the time we set the current minimum -B we were still
intending that you could run in a half meg or so SHMMAX allocation.
That's certainly history. Maybe we should target 2 meg as the rock
bottom minimum?
regards, tom lane
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