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Re: [HACKERS] WAL logging freezing

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>,"Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>,<pgsql-hackers(at)postgresql(dot)org>,<pgsql-patches(at)postgresql(dot)org>
Subject: Re: [HACKERS] WAL logging freezing
Date: 2006-10-31 00:58:40
Message-ID: 1162256320.11568.467.camel@silverbirch.site (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
On Mon, 2006-10-30 at 19:18 -0500, Tom Lane wrote:
> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> > I don't agree: If the truncation points are at 1 million, 2 million etc,
> > then if we advance the relvacuumxid from 1.2 million to 1.5 million,
> > then crash, the hints bits for that last vacuum are lost. Sounds bad,
> > but we have not truncated clog, so there is no danger.
> 
> You're still wrong though. 

Frequently, I'd say :-)

>  Suppose that VACUUM moves a particular rel's
> relvacuumxid from 1.9 to 2.1 million, but because this rel is not
> currently the oldest vacuumxid, it doesn't truncate clog.  Then we crash
> and lose hint bits, but not the relvacuumxid change.  Then VACUUM
> vacuums some other rel and advances its relvacuumxid from 1.9 to 2.1
> million --- but this time that *was* the globally oldest value, and now
> we think we can truncate clog at 2 million.  But the first rel might
> still have some unhinted xids around 1.9 million.

That was understood; in the above example I agree you need to flush. If
you don't pass a truncation point, you don't need to flush whether or
not you actually truncate. So we don't need to flush *every* time, so
IMHO we don't need to play safe and keep clog the size of an iceberg.

Anyway, if PITR is safe again, I'd like to sleep....zzzzzzzzzzzzzz

-- 
  Simon Riggs             
  EnterpriseDB   http://www.enterprisedb.com



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