Re: pg_receivexlog --status-interval add fsync feedback - Mailing list pgsql-hackers

> >> >>> If we remove --fsync-interval, resoponse time to user will not
> be
> >> delay.
> >> >>> Although, fsync will be executed multiple times in a short period.
> >> >>> And there is no way to solve the problem without
> >> >>> --fsync-interval, what
> >> >> should we do about it?
> >> >>
> >> >> I'm sorry, I didn't understand that.
> >> >
> >> > Here is an example.
> >> > When WAL is sent at 100ms intervals, fsync() is executed 10 times
> >> > per
> >> second.
> >> > If --fsync-interval is set by 1 second, we have to wait SQL
> >> responce(kind of making WAL record) for 1 second, though, fsync()
> >> won't be executed several times in 1 second.
> >> > I think --fsync-interval meets the demands of people who wants to
> >> restrain fsync() happens for several time in short period, but what
> >> do you think?
> >> > Is it ok to delete --fsync-interval ?
> >>
> >> I still don't see the problem.
> >>
> >> In synchronous mode, pg_receivexlog should have similar logic as
> >> walreceiver does. It should read as much WAL from the socket as it
> >> can without blocking, and fsync() and send reply after that. And also
> >> fsync whenever switching to new segment. If the master sends WAL
> >> every 100ms, then pg_recevexlog will indeed fsync() 10 times per
> >> second. There's nothing wrong with that.
> >>
> >> In asynchronous mode, only fsync whenever switching to new segment.
> >>
> >> >> Yeah. Or rather, add a new message type, to indicate the
> >> >> synchronous/asynchronous status.
> >> >
> >> > What kind of 'message type' we have to add ?
> >> >
> >> > Do we need to separate 'w' into two types ? synchronous and
> >> asynchronous ?
> >> >
> >> > OR
> >> >
> >> > Add a new message type, kind of 'notify synchronous', and inform
> >> > pg_receivexlog of synchronous status when it connect to the server.
> >>
> >> Better to add a new "notify" message type. And pg_recevexlog should
> >> be prepared to receive it at any time. The status might change on the
> >> fly, if the server's configuration is reloaded.
> >
> > Thanks for the reply.
> >
> >> In synchronous mode, pg_receivexlog should have similar logic as
> walreceiver does.
> >
> > OK. I understand that removing --fsync-interval has no problem.
> 
> +1 for adding something like --synchronous option. To me,
> it sounds walreceiver-compatible mode rather than synchronous.
> 
> >> Better to add a new "notify" message type. And pg_recevexlog should
> be prepared to receive it at any time. The status might change on the
> fly, if the server's configuration is reloaded.
> >
> > OK. I'll consider it.
> 
> I don't think that's good idea because it prevents us from using
> pg_receivexlog as async walreceiver (i.e., received WAL data is fsynced
> and feedback is sent back to the server soon, but transaction commit in
> the server doesn't wait for the feedback).

Thanks for the comment.

> it prevents us from using pg_receivexlog as async walreceiver.

Yes. If sync or async is switched by message,
in case pg_receivexlog is async, it won't work like a walreceiver.

User don't have to set options if synchronous status can be distinguished by message, though they cannot specify the
action.

> adding something like --synchronous option
Integrate required options for sync like above, is more appropriate ?

Regards,

--
Furuya Osamu