| From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Magnus Hagander <magnus(at)hagander(dot)net>, Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Michael Banck <michael(dot)banck(at)credativ(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Online enabling of checksums |
| Date: | 2018-04-06 18:18:00 |
| Message-ID: | 79b37247-fd2f-fd90-daf4-02e0fe8d7ad0@2ndquadrant.com |
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| Lists: | pgsql-hackers |
On 04/06/2018 08:13 PM, Andres Freund wrote:
> On 2018-04-06 19:59:17 +0200, Tomas Vondra wrote:
>> On 04/06/2018 07:46 PM, Andres Freund wrote:
>>>> 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.
>
> The buffer content lock itself doesn't generally give any such
> guarantee afaict, as it's required that the content lock is held in
> shared mode during IO. ProcessSingleRelationFork() happens to use
> exclusive mode (which could and possibly should be optimized), so
> that's probably sufficient from that end though.
>
Yes.
> I'm mainly disconcerted this isn't well discussed & documented.
>
Agreed, no argument here.
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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