"using previous checkpoint record at" maybe not the greatest idea?

From: Andres Freund <andres(at)anarazel(dot)de>
To: pgsql-hackers(at)postgresql(dot)org
Subject: "using previous checkpoint record at" maybe not the greatest idea?
Date: 2016-02-01 23:58:54
Message-ID: 20160201235854.GO8743@awork2.anarazel.de
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Hi,

currently if, when not in standby mode, we can't read a checkpoint
record, we automatically fall back to the previous checkpoint, and start
replay from there.

Doing so without user intervention doesn't actually seem like a good
idea. While not super likely, it's entirely possible that doing so can
wreck a cluster, that'd otherwise easily recoverable. Imagine e.g. a
tablespace being dropped - going back to the previous checkpoint very
well could lead to replay not finishing, as the directory to create
files in doesn't even exist.

As there's, afaics, really no "legitimate" reasons for needing to go
back to the previous checkpoint I don't think we should do so in an
automated fashion.

All the cases where I could find logs containing "using previous
checkpoint record at" were when something else had already gone pretty
badly wrong. Now that obviously doesn't have a very large significance,
because in the situations where it "just worked" are unlikely to be
reported...

Am I missing a reason for doing this by default?

Greetings,

Andres Freund

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