From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
---|---|
To: | Ants Aasma <ants(at)cybertec(dot)at> |
Cc: | sthomas(at)optionshouse(dot)com, Samrat Revagade <revagade(dot)samrat(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Inconsistent DB data in Streaming Replication |
Date: | 2013-04-08 16:38:22 |
Message-ID: | 20130408163822.GB29465@awork2.anarazel.de |
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Thread: | |
Lists: | pgsql-hackers |
On 2013-04-08 19:26:33 +0300, Ants Aasma wrote:
> On Mon, Apr 8, 2013 at 6:50 PM, Shaun Thomas <sthomas(at)optionshouse(dot)com> wrote:
> > On 04/08/2013 05:34 AM, Samrat Revagade wrote:
> >
> >> One solution to avoid this situation is have the master send WAL
> >> records to standby and wait for ACK from standby committing WAL files
> >> to disk and only after that commit data page related to this
> >> transaction on master.
> >
> >
> > Isn't this basically what synchronous replication does in PG 9.1+?
>
> Not exactly. Sync-rep ensures that commit success is not sent to the
> client before a synchronous replica acks the commit record. What
> Samrat is proposing here is that WAL is not flushed to the OS before
> it is acked by a synchronous replica so recovery won't go past the
> timeline change made in failover, making it necessary to take a new
> base backup to resync with the new master. I seem to remember this
> being discussed when sync rep was committed. I don't recall if the
> idea was discarded only on performance grounds or whether there were
> other issues too.
Thats not going to work for a fair number of reasons:
* wal is streamed *from disk* not from memory
* what if the local node crashes/restarts immediately? Then the standby
is farther ahead than the master.
* the performance implications of never writing data before flushing it
are pretty severe
* ...
So this doesn't seem to solve anything.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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