From: =?utf-8?Q?Dagfinn_Ilmari_Manns=C3=A5ker?= <ilmari@ilmari.org>
To: Simon Riggs <simon.riggs@enterprisedb.com>
Cc: Thomas Munro <thomas.munro@gmail.com>,  Julien Rouhaud
 <rjuju123@gmail.com>,  Ashutosh Sharma <ashu.coek88@gmail.com>,  Daniel
 Gustafsson <daniel@yesql.se>,  Stephen Frost <sfrost@snowman.net>,  Alvaro
 Herrera <alvherre@2ndquadrant.com>,  Andres Freund <andres@anarazel.de>,
  David Steele <david@pgmasters.net>,  Dmitry Dolgov
 <9erthalion6@gmail.com>,  Jakub Wartak <Jakub.Wartak@tomtom.com>,  Tom
 Lane <tgl@sss.pgh.pa.us>,  Tomas Vondra <tomas.vondra@2ndquadrant.com>,
  Tomas Vondra <tomas.vondra@enterprisedb.com>,  pgsql-hackers
 <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
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Date: Tue, 12 Apr 2022 16:57:41 +0100
In-Reply-To: 
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	(Simon Riggs's message of "Tue, 12 Apr 2022 14:58:17 +0100")
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Simon Riggs <simon.riggs@enterprisedb.com> writes:

> On Thu, 7 Apr 2022 at 08:46, Thomas Munro <thomas.munro@gmail.com> wrote:
>
>> With that... I've finally pushed the 0002 patch and will be watching
>> the build farm.
>
> This is a nice feature if it is safe to turn off full_page_writes.
>
> When is it safe to do that? On which platform?
>
> I am not aware of any released software that allows full_page_writes
> to be safely disabled. Perhaps something has been released recently
> that allows this? I think we have substantial documentation about
> safety of other settings, so we should carefully document things here
> also.

Our WAL reliability docs claim that ZFS is safe against torn pages:

https://www.postgresql.org/docs/current/wal-reliability.html:

    If you have file-system software that prevents partial page writes
    (e.g., ZFS), you can turn off this page imaging by turning off the
    full_page_writes parameter.

- ilmari