Re: finding changed blocks using WAL scanning

On Mon, Apr 22, 2019 at 11:20:43AM +0900, Michael Paquier wrote:
> On Sat, Apr 20, 2019 at 12:21:36AM -0400, Robert Haas wrote:
> > The segment size doesn't have much to do with it. If you make
> > segments bigger, you'll have to scan fewer larger ones; if you make
> > them smaller, you'll have more smaller ones. The only thing that
> > really matters is the amount of I/O and CPU required, and that doesn't
> > change very much as you vary the segment size.
>
> If you create the extra file when a segment is finished and we switch
> to a new one, then the extra work would happen for a random backend,
> and it is going to be more costly to scan a 1GB segment than a 16MB
> segment as a one-time operation, and less backends would see a
> slowdown at equal WAL data generated. From what I can see, you are
> not planning to do such operations when a segment finishes being
> written, which would be much better.

I think your point is that the 16MB is more likely to be in memory,
while the 1GB is less likely.

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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