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Re: Sync Rep Design

From: Hannu Krosing <hannu(at)2ndquadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Sync Rep Design
Date: 2010-12-31 07:50:07
Message-ID: 4D1D8B2F.5070001@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 30.12.2010 22:27, Robert Haas wrote:
> On Thu, Dec 30, 2010 at 2:04 PM, Simon Riggs<simon(at)2ndquadrant(dot)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


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