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From: Andres Freund <[email protected]>
To: Robert Haas <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: trying again to get incremental backup
Date: Wed, 14 Jun 2023 13:40:56 -0700
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+TgmoaF-TnXK9D5m3m1pTj6p5u0DiyzKuPZM1pqPGKzXhQw1w@mail.gmail.com>
References: <CA+TgmoYOYZfMCyOXFyC-P+-mdrZqm5pP2N7S-r0z3_402h9rsA@mail.gmail.com>
	<[email protected]>
	<CA+TgmoaF-TnXK9D5m3m1pTj6p5u0DiyzKuPZM1pqPGKzXhQw1w@mail.gmail.com>

Hi,

On 2023-06-14 16:10:38 -0400, Robert Haas wrote:
> On Wed, Jun 14, 2023 at 3:47 PM Andres Freund <[email protected]> wrote:
> > Could we just recompute the WAL summary for the [redo, end of chunk] for the
> > relevant summary file?
> 
> I'm not understanding how that would help. If we were going to compute
> a WAL summary on the fly rather than waiting for one to show up on
> disk, what we'd want is [end of last WAL summary that does exist on
> disk, redo].

Oh, right.


> But I'm not sure that's a great approach, because that LSN gap might be
> large and then we're duplicating a lot of work that the summarizer has
> probably already done most of.

I guess that really depends on what the summary granularity is. If you create
a separate summary every 32MB or so, recomputing just the required range
shouldn't be too bad.


> > FWIW, I like the idea of a special WAL record at that point, independent of
> > this feature. It wouldn't be a meaningful overhead compared to the cost of a
> > checkpoint, and it seems like it'd be quite useful for debugging. But I can
> > see uses going beyond that - we occasionally have been discussing associating
> > additional data with redo points, and that'd be a lot easier to deal with
> > during recovery with such a record.
> >
> > I don't really see a performance and concurrency angle right now - what are
> > you wondering about?
> 
> I'm not really sure. I expect Dilip would be happy to post his patch,
> and if you'd be willing to have a look at it and express your concerns
> or lack thereof, that would be super valuable.

Will do. Adding me to CC: might help, I have a backlog unfortunately :(.


Greetings,

Andres Freund

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