Message-ID: <5388A063.5000209@vmware.com>
Date: Fri, 30 May 2014 18:14:43 +0300
From: Heikki Linnakangas <hlinnakangas@vmware.com>
Organization: VMware
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To: Greg Stark <stark@mit.edu>
CC: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Andres Freund <andres@2ndquadrant.com>,
 Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>,
 Rajeev rastogi <rajeev.rastogi@huawei.com>, Ants Aasma <ants@cybertec.at>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>,
 Markus Wanner <markus@bluegap.ch>
Subject: Re: Proposal for CSN based snapshots
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On 05/30/2014 06:09 PM, Greg Stark wrote:
> On Fri, May 30, 2014 at 3:59 PM, Heikki Linnakangas
> <hlinnakangas@vmware.com> wrote:
>> If transaction A commits synchronously with commit LSN 1, and transaction B
>> commits asynchronously with commit LSN 2, B cannot become visible before A.
>> And we cannot acknowledge B as committed to the client until it's visible to
>> other transactions. That means that B will have to wait for A's commit
>> record to be flushed to disk, before it can return, even though it was an
>> asynchronous commit.
>
>
> I thought that's what happens now.
>
> What's more of a concern is synchronousl replication. We currently
> have a hack that makes transactions committed locally invisible to
> other transactions even though they've committed and synced to disk
> until the slave responds that it's received the transaction. (I think
> this is bogus personally, it just shifts the failure modes around. If
> we wanted to do it properly we would have to do two-phase commit.)

Yeah. To recap, the failure mode is that if the master crashes and 
restarts, the transaction becomes visible in the master even though it 
was never replicated.

> I guess it still works because we don't support having synchronous
> replication for just some transactions and not others. It would be
> nice to support that but I think it would mean making it work like
> local synchronous commit. It would only affect how long the commit
> blocks, not when other transactions see the committed data.

Actually, we do support that, see synchronous_commit=local.

- Heikki


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