Re: pg_receivexlog and feedback message

On Tue, Jun 5, 2012 at 11:44 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> On Tue, Jun 5, 2012 at 4:42 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>> On Tue, Jun 5, 2012 at 9:53 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>>> Right now, pg_receivexlog sets:
>>>                        replymsg->write = InvalidXLogRecPtr;
>>>                        replymsg->flush = InvalidXLogRecPtr;
>>>                        replymsg->apply = InvalidXLogRecPtr;
>>>
>>> when it sends it's status updates.
>>>
>>> I'm thinking it sohuld set replymsg->write = blockpos instad.
>>>
>>> Why? That way you can see in pg_stat_replication what has actually
>>> been received by pg_receivexlog - not just what we last sent. This can
>>> be useful in combination with an archive_command that can block WAL
>>> recycling until it has been saved to the standby. And it would be
>>> useful as a general monitoring thing as well.
>>>
>>> I think the original reason was that it shouldn't interefer with
>>> synchronous replication - but it does take away a fairly useful
>>> usecase...
>>
>> I think that not only replaymsg->write but also ->flush should be set to
>> blockpos in pg_receivexlog. Which allows pg_receivexlog to behave
>> as synchronous standby, so we can write WAL to both local and remote
>> synchronously. I believe there are some use cases for synchronous
>> pg_receivexlog.
>
> pg_receivexlog doesn't currently fsync() after every write. It only
> fsync():s complete files. So we'd need to set ->flush only at the end
> of a segment, right?

Yes.

Currently the status update is sent for each status interval. In sync
replication, transaction has to wait for a while even after pg_receivexlog
has written or flushed the WAL data.

So we should add new option which specifies whether pg_receivexlog
sends the status packet back as soon as it writes or flushes the WAL
data, like the walreceiver does?

Regards,

--
Fujii Masao