Re: Synchronous Log Shipping Replication

Fujii Masao wrote:
> On Mon, Sep 8, 2008 at 8:44 PM, Markus Wanner <markus(at)bluegap(dot)ch> wrote:
> >> Merge into WAL writer?
> >
> > Uh.. that would mean you'd loose parallelism between WAL writing to disk and
> > WAL shipping via network. That does not sound appealing to me.
>
> That depends on the order of WAL writing and WAL shipping.
> How about the following order?
>
> 1. A backend writes WAL to disk.
> 2. The backend wakes up WAL sender process and sleeps.
> 3. WAL sender process does WAL shipping and wakes up the backend.
> 4. The backend issues sync command.

I am confused why this is considered so complicated. Having individual
backends doing the wal transfer to the slave is never going to work
well.

I figured we would have a single WAL streamer that continues advancing
forward in the WAL file, streaming to the standby. Backends would
update a shared memory variable specifying how far they want the wal
streamer to advance and send a signal to the wal streamer if necessary.
Backends would monitor another shared memory variable that specifies how
far the wal streamer has advanced.

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

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