From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Jeff Davis <pgsql@j-davis.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Greg Stark <stark@mit.edu>
Cc: Andres Freund <andres@2ndquadrant.com>
Cc: Rajeev rastogi <rajeev.rastogi@huawei.com>
Cc: Markus Wanner <markus@bluegap.ch>
Cc: Ants Aasma <ants@cybertec.at>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: obartunov <obartunov@postgrespro.ru>
Cc: Teodor Sigaev <teodor@postgrespro.ru>
Subject: Re: Proposal for CSN based snapshots
Date: Fri, 24 Jul 2015 20:55:20 +0200
Message-ID: <20150724185520.GG21169@awork2.anarazel.de> (raw)
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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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To: pgsql-hackers@postgresql.org
Cc: andres@anarazel.de, robertmhaas@gmail.com, simon@2ndquadrant.com, a.korotkov@postgrespro.ru, hlinnakangas@vmware.com, dilipbalaut@gmail.com, pgsql@j-davis.com, alvherre@2ndquadrant.com, stark@mit.edu, andres@2ndquadrant.com, rajeev.rastogi@huawei.com, markus@bluegap.ch, ants@cybertec.at, bruce@momjian.us, obartunov@postgrespro.ru, teodor@postgrespro.ru
Subject: Re: Proposal for CSN based snapshots
In-Reply-To: <20150724185520.GG21169@awork2.anarazel.de>
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