Date: Fri, 24 Jul 2015 20:55:20 +0200
From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Simon Riggs <simon@2ndquadrant.com>,
 Alexander Korotkov <a.korotkov@postgrespro.ru>,
 Heikki Linnakangas <hlinnakangas@vmware.com>,
 Dilip Kumar <dilipbalaut@gmail.com>, Jeff Davis <pgsql@j-davis.com>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>,
 Alvaro Herrera <alvherre@2ndquadrant.com>,
 Greg Stark <stark@mit.edu>, Andres Freund <andres@2ndquadrant.com>,
 Rajeev rastogi <rajeev.rastogi@huawei.com>,
 Markus Wanner <markus@bluegap.ch>, Ants Aasma <ants@cybertec.at>,
 Bruce Momjian <bruce@momjian.us>, obartunov <obartunov@postgrespro.ru>,
 Teodor Sigaev <teodor@postgrespro.ru>
Subject: Re: Proposal for CSN based snapshots
Message-ID: <20150724185520.GG21169@awork2.anarazel.de>
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On 2015-07-24 14:21:39 -0400, Robert Haas wrote:
> - Mixing synchronous_commit=on and synchronous_commit=off won't work
> as well, because if the LSN ordering of commit records matches the
> order in which transactions become visible, then an async-commit
> transaction can't become visible before a later sync-commit
> transaction.  I expect we might just decide we can live with this, but
> it's worth discussing in case people feel otherwise.

I'm not following this anymore. Even though a couple months back I
apparently did. Heikki's description of the problem was:

On 2014-05-30 17:59:23 +0300, Heikki Linnakangas wrote:
> One thorny issue came up in discussions with other hackers on this in PGCon:
> 
> When a transaction is committed asynchronously, it becomes visible to other
> backends before the commit WAL record is flushed. With CSN-based snapshots,
> the order that transactions become visible is always based on the LSNs of
> the WAL records. This is a problem when there is a mix of synchronous and
> asynchronous commits:
> 
> 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.


Afacs the xlog insertion order isn't changed relevantly by the patch?

Right now the order for sync commit = on is:

XactLogCommitRecord();
XLogFlush();
TransactionIdCommitTree();
ProcArrayEndTransaction();

and for sync commit = off it's:
XactLogCommitRecord();
TransactionIdAsyncCommitTree();
ProcArrayEndTransaction();

with the CSN patch it's:

sync on:
TransactionIdSetCommitting();
XactLogCommitRecord();
XLogFlush(XactLastRecEnd);
TransactionIdCommitTree();

sync off:
TransactionIdSetCommitting();
XactLogCommitRecord();
TransactionIdCommitTree();

How does this cause the above described problem? Yes, mixing sync/async
commits can give somewhat weird visibility semantics, but that's not
really new, is it?

Regards,

Andres


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