From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Jakub Wartak <Jakub.Wartak@tomtom.com>
Subject: Re: Background writer and checkpointer in crash recovery
Date: Sat, 29 Aug 2020 20:39:08 -0400
Message-ID: <3750591.1598747948@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+hUKGJ8NRsqgkZEnsnRc2MFROBV-jCnacbYvtpptK2A9YYp9Q@mail.gmail.com>
References: <CA+hUKGJ8NRsqgkZEnsnRc2MFROBV-jCnacbYvtpptK2A9YYp9Q@mail.gmail.com>
Thomas Munro <thomas.munro@gmail.com> writes:
> Once we had the checkpointer running, we could also consider making
> the end-of-recovery checkpoint optional, or at least the wait for it
> to complete. I haven't shown that in this patch, but it's just
> different checkpoint request flags and possibly an end-of-recovery
> record. What problems do you foresee with that? Why should we have
> "fast" promotion but not "fast" crash recovery?
I think that the rationale for that had something to do with trying
to reduce the costs of repeated crashes. If you've had one crash,
you might well have another one in your near future, due to the same
problem recurring. Therefore, avoiding multiple replays of the same WAL
is attractive; and to do that we have to complete a checkpoint before
giving users a chance to crash us again.
I'm not sure what I think about your primary proposal here. On the
one hand, optimizing crash recovery ought to be pretty darn far down
our priority list; if it happens often enough for performance to be
a consideration then we have worse problems to fix. Also, at least
in theory, not running the bgwriter/checkpointer makes for fewer moving
parts and thus better odds of completing crash recovery successfully.
On the other hand, it could be argued that running without the
bgwriter/checkpointer actually makes recovery less reliable, since
that's a far less thoroughly-tested operating regime than when they're
active.
tl;dr: I think this should be much less about performance than about
whether we successfully recover or not. Maybe there's still a case for
changing in that framework, though.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, thomas.munro@gmail.com, simon@2ndquadrant.com, hlinnaka@iki.fi, Jakub.Wartak@tomtom.com
Subject: Re: Background writer and checkpointer in crash recovery
In-Reply-To: <3750591.1598747948@sss.pgh.pa.us>
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