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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Simon Riggs <simon.riggs@enterprisedb.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Julien Rouhaud <rjuju123@gmail.com>
Cc: Ashutosh Sharma <ashu.coek88@gmail.com>
Cc: Daniel Gustafsson <daniel@yesql.se>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: David Steele <david@pgmasters.net>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Jakub Wartak <Jakub.Wartak@tomtom.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
Date: Tue, 12 Apr 2022 18:07:20 +0200
Message-ID: <3f4d65c8-ad61-9f57-d5ad-6c1ea841e471@enterprisedb.com> (raw)
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On 4/12/22 17:46, Simon Riggs wrote:
> On Tue, 12 Apr 2022 at 16:41, Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>>
>> On 4/12/22 15:58, Simon Riggs wrote:
>>> 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.
>>>
>>
>> I don't see why/how would an async prefetch make FPW unnecessary. Did
>> anyone claim that be the case?
> 
> Other way around. FPWs make prefetch unnecessary.
> Therefore you would only want prefetch with FPW=off, AFAIK.
> 
> Or put this another way: when is it safe and sensible to use
> recovery_prefetch != off?
> 

That assumes the FPI stays in memory until the next modification, and
that can be untrue for a number of reasons. Long checkpoint interval
with enough random accesses in between is a nice example. See the
benchmarks I did a year ago (regular pgbench).

Or imagine a r/o replica used to run analytics queries, that access so
much data it evicts the buffers initialized by the FPI records.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company





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