Message-ID: <4D1D8B2F.5070001@2ndquadrant.com>
Date: Fri, 31 Dec 2010 08:50:07 +0100
From: Hannu Krosing <hannu@2ndquadrant.com>
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To: Robert Haas <robertmhaas@gmail.com>
CC: Simon Riggs <simon@2ndquadrant.com>,
	Stefan Kaltenbrunner <stefan@kaltenbrunner.cc>,
	pgsql-hackers@postgresql.org
Subject: Re: Sync Rep Design
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	<AANLkTi=SvN2KQ1Tzh-tRKcXD0i2FyWFBbbR=kXXdepqN@mail.gmail.com>
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On 30.12.2010 22:27, Robert Haas wrote:
> On Thu, Dec 30, 2010 at 2:04 PM, Simon Riggs<simon@2ndquadrant.com>  wrote:
>> synchronous_replication (boolean)
>>         Specifies whether transaction commit will wait for WAL records
>>         to be replicated before the command returns a "success"
>>         indication to the client.
> The word "replicated" here could be taken to mean different things,
> most obviously:
>
> - slave has received the WAL
> - slave has fsync'd the WAL
> - slave has applied the WAL
Perhaps the level of "replication guarantee" should be decided on the 
slave side, by
having a configuration parameter there

report_as_replicated = received|written_to_disk|fsynced|applied

for different types of hosts may have wildly different guarantees and 
performance
parameters for these. One could envision a WAL-archive type "standby" 
which is
there for data persistence only will and never "apply" WAL.

of couse we could put a bitmap in the status update messages from slave 
and have
some quorum on options on master for when the data is "in sync", say 
"need 5 received
or (1 applied and 1 fsynced)", but I am pretty sure that trying to get 
anywhere with this
before applying the basic sync rep patch would push back sync rep to at 
least 9.2 if not 9.5

---------------------
Hannu Krosing