Message-ID: <5191B73C.9040100@agliodbs.com>
Date: Mon, 13 May 2013 21:02:04 -0700
From: Josh Berkus <josh@agliodbs.com>
Organization: PostgreSQL Experts Inc.
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To: Andres Freund <andres@2ndquadrant.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Setting vacuum_freeze_min_age really low
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HAndres,

> Well. For one you haven't proven that the changed setting actually
> improves performance. So the comparison isn't really valid. We will

I agree that I haven't proven this yet, but that doesn't make it
invalid.  Just unproven.

I agree that performance testing is necessary ... and the kind of
performance testing which generated freeze activity, which makes it harder.

> I think you're missing the fact that we don't neccessarily dirty pages,
> just because vacuum visits them. In a mostly insert workload its not
> uncommon that vacuum doesn't change anything. In many scenarios the

Hmmm.  But does vacuum visit the pages anyway, in that case?

> b) freezing tuples requires a xlog_heap_freeze wal record to be
>    emitted. If we don't freeze, we don't need to emit it.

Oh, that's annoying.

> I think I have said that before, but anyway: I think as long as we need
> to regularly walk the whole relation for correctness there isn't much
> hope to get this into an acceptable state. If we would track the oldest
> xid in a page in a 'freeze map' we could make much of this more
> efficient and way more scalable to bigger data volumes.

Yeah, or come up with some way to eliminate freezing entirely.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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