From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jim C. Nasby <jim@nasby.net>
Cc: ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Dead Space Map version 2
Date: Tue, 27 Feb 2007 09:51:09 +0000
Message-ID: <45E3FF0D.2050104@enterprisedb.com> (raw)
In-Reply-To: <12631.1172555721@sss.pgh.pa.us>
References: <20070227113655.6381.ITAGAKI.TAKAHIRO@oss.ntt.co.jp>
<20070227051144.GK29041@nasby.net>
<12631.1172555721@sss.pgh.pa.us>
Tom Lane wrote:
> The main problem with the levels proposed by Takahiro-san is that any
> transition from FROZEN to not-FROZEN *must* be exactly recovered,
> because vacuum will never visit an allegedly frozen page at all. This
> appears to require WAL-logging DSM state changes, which is a pretty
> serious performance hit.
I doubt it would be a big performance hit. AFAICS, all the information
needed to recover the DSM is already written to WAL, so it wouldn't need
any new WAL records.
> I'd be happier if the DSM content could be
> treated as just a hint. I think that means not trusting it for whether
> a page is frozen to the extent of not needing vacuum even for
> wraparound. So I'm inclined to propose that there be only two states
> (hence only one DSM bit per page): page needs vacuum for space recovery,
> or not. Vacuum for XID wraparound would have to hit every page
> regardless.
If we don't have a frozen state, we can't use the DSM to implement
index-only scans. Index-only scans will obviously require a lot more
work than just the DSM, but I'd like to have a solution that enables it
in the future.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-performance@postgresql.org
Cc: heikki@enterprisedb.com, tgl@sss.pgh.pa.us, jim@nasby.net, itagaki.takahiro@oss.ntt.co.jp, pgsql-hackers@postgresql.org
Subject: Re: Dead Space Map version 2
In-Reply-To: <45E3FF0D.2050104@enterprisedb.com>
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