Horton

Re: Online enabling of checksums

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Magnus Hagander <magnus@hagander.net>, Andrey Borodin <x4mmm@yandex-team.ru>, Heikki Linnakangas <hlinnaka@iki.fi>, Daniel Gustafsson <daniel@yesql.se>, Michael Banck <michael.banck@credativ.de>, Robert Haas <robertmhaas@gmail.com>, Greg Stark <stark@mit.edu>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-04-06T17:59:17Z
Lists: pgsql-hackers 

On 04/06/2018 07:46 PM, Andres Freund wrote:
> On 2018-04-06 19:40:59 +0200, Tomas Vondra wrote:
>> In any case, I wouldn't call LockBufHdr/UnlockBufHdr a "side channel"
>> interlock. It's a pretty direct and intentional interlock, I think.
> 
> I mean it's a side-channel as far as DataChecksumsNeedWrite() is 
> concerned. You're banking on all callers using a barrier implying 
> operation around it.

Ah, OK.

> 
>> Sure. But what would that be? I can't think of anything. A process that
>> modifies a buffer (or any other piece of shared state) without holding
>> some sort of lock seems broken by default.
> 
> You can quite possibly already *hold* a lock if it's not an exclusive
> one.
> 

Sure, but if you're holding the buffer lock when the checksum version is
changed, then the checksumhelper is obviously not running yet. In which
case it will update the checksum on the buffer later.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. Online enabling and disabling of data checksums

  2. Deactive flapping checksum isolation tests.

  3. Add support for coordinating record typmods among parallel workers.