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Re: AW: WAL & RC1 status

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Zeugswetter Andreas SB <ZeugswetterA(at)Wien(dot)Spardat(dot)at>
Cc: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: AW: WAL & RC1 status
Date: 2001-03-05 16:13:52
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Lists: pgsql-hackers
Zeugswetter Andreas SB  <ZeugswetterA(at)Wien(dot)Spardat(dot)at> writes:
>> At least one of my concerns (single point of failure) would require a
>> change to the layout of pg_control, which would force initdb anyway.

> Was that the "only one checkpoint back in time in pg_control" issue ?


> One issue about too many checkpoints in pg_control, is that you then
> need to keep more logs, and in my pgbench tests the log space was a
> real issue, even for the one checkpoint case. I think a utility to
> recreate a busted pg_control would add a lot more stability, than one
> more checkpoint in pg_control.

Well, there is a big difference between 1 and 2 checkpoints stored in
pg_control.  I don't intend to go further than 2.  But I disagree about
a log-reset utility being more useful than an extra checkpoint.  The
utility would be for manual recovery after a disaster, and it wouldn't
offer 100% recovery: you couldn't be sure that the last few transactions
had been applied atomically, ie, all or none.  (Perhaps pg_log got
updated to show them committed, but not all of their tuple changes made
it to disk; how will you know?)  If you can back up to the prior
checkpoint and then roll forward, you *do* have a shot at guaranteeing
a consistent database state after loss of the primary checkpoint.

> We should probably have additional criteria to time, that can trigger a 
> checkpoint, like N logs filled since last checkpoint.

Perhaps.  I don't have time to work on that now, but we can certainly
improve the strategy in future releases.

			regards, tom lane

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