From: Bruce Momjian <bruce@momjian.us>
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: yigong hu <yigongh@gmail.com>
Cc: splarv@ya.ru
Cc: pgsql-docs@lists.postgresql.org
Subject: Re: explanation for random_page_cost is outdated
Date: Mon, 27 Apr 2020 12:16:30 -0400
Message-ID: <20200427161630.GB21731@momjian.us> (raw)
In-Reply-To: <CAFj8pRA35t++NeQ2mbT=3O9=o7M0hrsRZUmPOCRErZR7Am5A_A@mail.gmail.com>
References: <20200319205646.GB28431@momjian.us>
<72492300-BFD4-4CCF-9136-20CC41A18A10@ya.ru>
<CAOxFffcourucFqSk+tZA13ErS3XRYkDy6EeaPff4AvHGiEEuug@mail.gmail.com>
<CAFj8pRA35t++NeQ2mbT=3O9=o7M0hrsRZUmPOCRErZR7Am5A_A@mail.gmail.com>
On Mon, Apr 27, 2020 at 06:02:41AM +0200, Pavel Stehule wrote:
>
>
> ne 26. 4. 2020 v 21:25 odesílatel yigong hu <yigongh@gmail.com> napsal:
>
> Sorry to hijack the thread, I also recently have similar observation that
> the statement about random_page_cost on SSD is ambiguous. The current
> document says that
>
> > Storage that has a low random read cost relative to sequential, e.g.
> solid-state drives, might also be better modeled with a lower value for
> random_page_cost.
>
> However, this statement does not clarify what values might be good. For
> some workload, the default value 4.0 would cause bad performance and
> lowering random_page_cost to a value 3.0 or 2.0 does not solve the
> performance problem. Only when the random_page_cost is lowered to below 1.2
> will the bad performance be mitigated. Thus, I would suggest elaborating on
> this description further as:
>
> > Storage that has a low random read cost relative to sequential, e.g.
> solid-state drives, might also be better modeled with a value that is close
> to 1 for random_page_cost.
>
>
> I depends on estimation. Lot of people use random_page_cost as fix of broken
> estimation. Then configures this value to some strange values. Lot of other
> queries with good estimation can be worse then.
I have been recommending 1.1 as a value for random_page_cost for SSDs
for years, and I think it would be helpful to suggest that value, so doc
patch attached.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
Attachments:random.diff (text/x-diff, 979B) downloaddiff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgmlnew file mode 100644index a14df06..38ae273
*** a/doc/src/sgml/config.sgml
--- b/doc/src/sgml/config.sgml
*************** ANY <replaceable class="parameter">num_s
*** 4915,4921 ****
the database is smaller than the total server memory, decreasing
random_page_cost can be appropriate. Storage that has a low random
read cost relative to sequential, e.g. solid-state drives, might
! also be better modeled with a lower value for random_page_cost.
</para>
<tip>
--- 4915,4922 ----
the database is smaller than the total server memory, decreasing
random_page_cost can be appropriate. Storage that has a low random
read cost relative to sequential, e.g. solid-state drives, might
! also be better modeled with a lower value for random_page_cost,
! e.g., <literal>1.1</literal>.
</para>
<tip>
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To: pgsql-docs@postgresql.org
Cc: bruce@momjian.us, pavel.stehule@gmail.com, yigongh@gmail.com, splarv@ya.ru, pgsql-docs@lists.postgresql.org
Subject: Re: explanation for random_page_cost is outdated
In-Reply-To: <20200427161630.GB21731@momjian.us>
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