From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: Greg Sabino Mullane <htamfids@gmail.com>,
        Dagfinn Ilmari =?utf-8?Q?Manns=C3=A5ker?= <ilmari@ilmari.org>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Changing the default random_page_cost value
In-reply-to: <ZyPB3KUjsL88h0h4@momjian.us>
References: <CAKAnmmK_nSPYr53LobUwQD59a-8U9GEC3XGJ43oaTYJq5nAOkw@mail.gmail.com> <877caxaxt6.fsf@wibble.ilmari.org> <CAKAnmmKxOq8i4JCfc6AzThuetLPenGaOQu25m0EeOhanaQdhLw@mail.gmail.com> <Zw2J5g08inev3tzT@momjian.us> <CAKAnmm+0UEcPZ0oC+7att-EAU7jXtkPJ1QcJ4v=sWzdkU92r+Q@mail.gmail.com> <ZyPB3KUjsL88h0h4@momjian.us>
Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
	message dated "Thu, 31 Oct 2024 13:43:56 -0400"
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Date: Thu, 31 Oct 2024 13:53:16 -0400
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Bruce Momjian <bruce@momjian.us> writes:
> On Thu, Oct 24, 2024 at 08:01:11PM -0400, Greg Sabino Mullane wrote:
>> Okay, but we have no documented logic on why 4.0 was chosen either. :)

> Uh, we do, and it is in the docs:

>         Random access to mechanical disk storage is normally much more e=
xpensive
>         than four times sequential access.  However, a lower default is =
used
>         (4.0) because the majority of random accesses to disk, such as i=
ndexed
>         reads, are assumed to be in cache.  The default value can be tho=
ught of
>         as modeling random access as 40 times slower than sequential, wh=
ile
>         expecting 90% of random reads to be cached.

Meh.  Reality is that that is somebody's long-after-the-fact apologia
for a number that was obtained by experimentation.

			regards, tom lane