To: Bruce Momjian <pgman@candle.pha.pa.us>
Cc: Simon Riggs <simon@2ndquadrant.com>, markw@osdl.org, kn@mgnet.de,
	pgsql-patches@postgresql.org
Subject: Re: [HACKERS] Point in Time Recovery 
In-reply-to: <200407192024.i6JKOrO16297@candle.pha.pa.us> 
References: <200407192024.i6JKOrO16297@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Mon, 19 Jul 2004 16:24:52 -0400"
Date: Mon, 19 Jul 2004 17:24:06 -0400
Message-ID: <29939.1090272246@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> we need to check to see which one has a WAL eof-of-segment marker (we
> have on of those, right?).

No, we don't.

> I think I see a solution. We are going to create a file during backup so
> we know the wal offsets and xids.  If we see that file, we know either
> we have a restore of a backup or they currently running a backup.

... or the last backup attempt failed, but they forgot to remove the
file it left.  Or we are doing crash recovery after the system lost
power while a backup was running.  Or half a dozen other obvious scenarios.

> If we tell them not to restore while a backup is running (seems pretty
> obvious) we can then delete pg_xlog when the backup wal offset file
> exists.  In other cases, we know the WAL files are valid to use.

We're not deleting pg_xlog, period.  IMHO it's too dangerous even to
have such a function in the code.

My original suggestion was to *replace* individual xlog files with data
extracted from archive, and only after determining that the archive
indeed has a copy of that particular file (and we can fetch it).
This at least has a fighting chance of not losing information.  Wiping
pg_xlog in toto on the basis of a guess about the system status is just
a form of russian roulette.  Sooner or later you will wipe some xlog
files that you can't get back from archive.

			regards, tom lane