Re: Synchronous replication - patch status inquiry

On Tue, 2010-09-07 at 11:41 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > On Tue, 2010-09-07 at 10:47 -0400, Tom Lane wrote:
> >> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> >>> The WAL is sent from master to standby in 8192 byte chunks, frequently
> >>> including multiple commits. From standby, one reply per chunk. If we
> >>> need to wait for apply while nothing else is received, we do.
> >>
> >> That premise is completely false. SR does not send WAL in page units.
> >> If it did, it would have the same performance problems as the old
> >> WAL-file-at-a-time implementation, just with slightly smaller
> >> granularity.
>
> > There's no dependence on pages in that proposal, so don't understand.
>
> Oh, well you certainly didn't explain it well then.
>
> What I *think* you're saying is that the slave doesn't send per-commit
> messages, but instead processes the WAL as it's received and then sends
> a heres-where-I-am status message back upstream immediately before going
> to sleep waiting for the next chunk. That's fine as far as the protocol
> goes, but I'm not convinced that it really does all that much in terms
> of improving performance. You still have the problem that the master
> has to fsync its WAL before it can send it to the slave. Also, the
> slave won't know whether it ought to fsync its own WAL before replying.

Yes, apart from last sentence. Please wait for the code.

--
Simon Riggs www.2ndQuadrant.com
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