Re: Support for N synchronous standby servers - take 2

On 5 April 2016 at 11:18, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:

> > 1. Header comments in syncrep.c need changes, not just additions.
>
> Okay, will consider this later. And I'd appreciate if you elaborate what
> changes are necessary specifically.

Some of the old header comments are now wrong.

> > 2. We need tests to ensure that k >=1 and k<=N
>
> The changes to replication test framework was included in the patch before,
> but I excluded it from the patch because I'd like to commit the core part
> of
> the patch first. Will review the test part later.

I meant tests of setting the parameters, not tests of the feature itself.

> >
> > 3. There should be a WARNING if k == N to say that we don't yet provide a
> > level to give Apply consistency. (I mean if we specify 2 (n1, n2) or
> 3(n1,
> > n2, n3) etc
>
> Sorry I failed to get your point. Could you tell me what Apply consistency
> and why we cannot provide it when k = N?
>
> > 4. How does it work?
> > It's pretty strange, but that isn't documented anywhere. It took me a
> while
> > to figure it out even though I know that code. My thought is its a lot
> > slower than before, which is a concern when we know by definition that k
> >=2
> > for the new feature. I was going to mention the fact that this code only
> > needs to be executed by standbys mentioned in s_s_n, so we can avoid
> > overhead and contention for async standbys (But Masahiko just mentioned
> that
> > also).
>
> Unless I'm missing something, the patch already avoids the overhead
> of async standbys. Please see the top of SyncRepReleaseWaiters().
> Since async standbys exit at the beginning of SyncRepReleaseWaiters(),
> they don't need to perform any operations that the patch adds
> (e.g., find out which standbys are synchronous).
>

I was thinking about the overhead of scanning through the full list of
WALSenders for each message, when it is a sync standby.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services