From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Andres Freund <andres@2ndquadrant.com>,
 PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
In-reply-to: <20140620181104.GF29143@momjian.us>
References: <20140619020633.GA20469@momjian.us>
 <20140619015205.GD3806@momjian.us> <20140619014034.GC3806@momjian.us>
 <20140619220425.GQ18688@eldon.alvh.no-ip.org>
 <20140619221240.GC27747@eldon.alvh.no-ip.org>
 <20140620171915.GC29143@momjian.us>
 <20140620173351.GV18688@eldon.alvh.no-ip.org> <8519.1403286102@sss.pgh.pa.us>
 <20140620181104.GF29143@momjian.us>
Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
 message dated "Fri, 20 Jun 2014 14:11:04 -0400"
Date: Fri, 20 Jun 2014 14:24:55 -0400
Message-ID: <9690.1403288695@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-bugs-owner@postgresql.org

Bruce Momjian <bruce@momjian.us> writes:
> On Fri, Jun 20, 2014 at 01:41:42PM -0400, Tom Lane wrote:
>> Yeah.  In my mind, at least, there's been a TODO for some time for
>> pg_resetxlog to make sure that pg_clog and the other SLRU-like files
>> get populated appropriately when you tell it to change those counters.
>> You have to have a segment file at the right place or startup fails.

> Well, if we are going to do this, we had better do all cases or the
> behavior will be quite surprising, and when doing disaster recovery,
> surprises are not good.  I am a little worried that removing files as
> part of pg_resetxlog might cause data to be lost as users try to figure
> things out.

Huh?  The basic charter of pg_resetxlog is to throw away files, ie, all
of your WAL.

But in this case what we're talking about is a secondary function, namely
the ability to move the XID, MXID, etc counter values.  Up to now, if you
did that, it was on your head to manually create appropriate SLRU segment
files.  It's certainly less error-prone, not more so, if the code were
to take care of that for you.

			regards, tom lane


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