Re: Standbys which don't synch to disk?

On Thu, May 12, 2011 at 3:48 AM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> Robert,
>
>> > That WAL has effectively disappeared from the
>> > master, but is still present on the slave.  Now the master comes up
>> > and starts processing read-write transactions again, and generates a
>> > new and different 1kB of WAL.  Hilarity ensues, because the two
>> > machines are now out of step with each other.
>
> Yeah, you'd need some kind of instant failover and STONITH.  That is,
> any interruption on the master would be a failover situation.  While
> that seems conceivable for crashes, consider that a planned restart of
> the master might be an issue, and an OOM-kill would certainly be.
>
>> > You could possibly fix this by making provision for the master to
>> > connect to the slave on start-up and stream WAL "backwards" from slave
>> > to master.  That'd be pretty spiffy.
>
> Ouch, now you're making my head hurt.

I believe many people who use SR with a clusterware would do failover
instead of restarting the master when it crashes. So I don't think it's
bad idea to allow them to use the stream-WAL-from-buffers feature
with self-responsibility. It's the same thing as we can specify fsync=off
or full_page_writes=off.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center