Re: Synchronization levels in SR

On Thu, 2010-05-27 at 11:28 +0900, Fujii Masao wrote:
> On Wed, May 26, 2010 at 10:20 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> > On Wed, 2010-05-26 at 18:52 +0900, Fujii Masao wrote:
> >
> >> I guess that dropping the support of #3 doesn't reduce complexity
> >> since the code of #3 is almost the same as that of #2. Like
> >> walreceiver sends the ACK after receiving the WAL in #2 case, it has
> >> only to do the same thing after the WAL flush.
> >
> > Hmm, well the code for #3 is similar also to the code for #4. So if you
> > do #2, its easy to do #2, #3 and #4 together.
>
> No. #4 requires the way of prompt communication between walreceiver and
> startup process, but #2 and #3 not. That is, in #4, walreceiver has to
> wake the startup process up as soon as it has flushed WAL. OTOH, the
> startup process has to wake walreceiver up as soon as it has replayed
> WAL, to request it to send the ACK to the master. In #2 and #3, the
> prompt communication from walreceiver to startup process, i.e., changing
> the poll loop in the startup process would also be useful for the data
> to be visible immediately on the standby. But it's not required.

You need to pass WAL promptly on primary from backend to WALSender.
Whatever mechanism you use can also be reused symmetrically on standby
to provide #4. So not a problem.

> > The comment is about whether having #3 makes sense from a user interface
> > perspective. It's easy to add options, but they must have useful
> > meaning.
>
> #3 would be useful for people wanting further robustness. In #2,
> when simultaneous power failure on the master and the standby,
> and concurrent disk crash on the master happen, transaction whose
> "success" indicator has been returned to a client might be lost.
> #3 can avoid such a critical situation. This is one of reasons that
> DRBD supports "Protocol C", I think.

Which few people use it, or if they do its because DRBD didn't
originally support multiple standbys. Not worth emulating IMHO.

--
Simon Riggs www.2ndQuadrant.com