| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, Steve Kehlet <steve(dot)kehlet(at)gmail(dot)com>, Forums postgresql <pgsql-general(at)postgresql(dot)org>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Re: [GENERAL] 9.4.1 -> 9.4.2 problem: could not access status of transaction 1 |
| Date: | 2015-06-02 21:22:55 |
| Message-ID: | 20150602212255.GW30287@alap3.anarazel.de |
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| Thread: | |
| Lists: | pgsql-general pgsql-hackers |
> > Hm. If GetOldestMultiXactOnDisk() gets the starting point by scanning
> > the disk it'll always get one at a segment boundary, right? I'm not sure
> > that's actually ok; because the value at the beginning of the segment
> > can very well end up being a 0, as MaybeExtendOffsetSlru() will have
> > filled the page with zeros.
> >
> > I think that should be harmless, the worst that can happen is that
> > oldestOffset errorneously is 0, which should be correct, even though
> > possibly overly conservative, in these cases.
>
> Uh oh. That seems like a real bad problem for this approach. What
> keeps that from being the opposite of too conservative? There's no
> "safe" value in a circular numbering space.
I think it *might* (I'm really jetlagged) be fine because that'll only
happen after a upgrade from < 9.3. And in that case we initialize
nextOffset to 0. That ought to safe us?
Greetings,
Andres Freund
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