Re: Streaming replication

> On 31/03/15 12:45, Tatsuo Ishii wrote:
>> In the doc:
>> 
>> 25.2.5. Streaming Replication
>> :
>> The standby connects to the primary, which streams WAL records to the
>> standby as they're generated, without waiting for the WAL file to be
>> filled.
>> 
>> This seems to claim that walsender sends WAL files which has not been
>> fsync'd yet. However, in walsender.c:
>> 
>>         /*
>>          * Streaming the current timeline on a master.
>>          *
>>          * Attempt to send all data that's already been written out and
>>          * fsync'd to disk.  We cannot go further than what's been written out
>>          * given the current implementation of XLogRead().  And in any case
>>          * it's unsafe to send WAL that is not securely down to disk on the
>>          * master: if the master subsequently crashes and restarts, slaves
>>          * must not have applied any WAL that gets lost on the master.
>>          */
>>     
>> This one says walsender sends WAL records as long as there are
>> fsync'd.
>> 
>> Am I missing something?
>> 
>>
> 
> I think the docs are trying to say that streaming replication doesn't
> wait for a (16MB) WAL *file* to be filled. but sends each (fsync'd) WAL
> record. I had to reread it several times too :-)

Thanks for the explanation. That makes sense. The docs definitely has
a room for improvement.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp