Re: [PATCH] 2PC state files on shared memory

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [PATCH] 2PC state files on shared memory
Date: 2009-08-08 15:29:05
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Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> Tom Lane wrote:
>> Quite aside from that, the fixed size of shared memory makes this seem
>> pretty impractical.

> Most state files are small. If one doesn't fit in the area reserved for
> this, it's written to disk as usual. It's just an optimization.

What evidence do you have for that assumption? And what's "small" anyway?
I think setting the size parameter for this would be a frightfully
difficult problem; the fact that average installations wouldn't use it
doesn't make that any better for those who would. After our bad
experiences with fixed-size FSM, I'm pretty wary of introducing new
fixed-size structures that the user is expected to figure out how to

> I'm a bit disappointed by the performance gains. I would've expected
> more, given a decent battery-backed-up cache to buffer the WAL fsyncs.
> But it looks like they're still causing the most overhead, even with a
> battery-backed-up cache.

If you can't demonstrate order-of-magnitude speedups, I think we
shouldn't touch this.

regards, tom lane

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