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From: Andres Freund <[email protected]>
To: Bruce Momjian <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: Increase default maintenance_io_concurrency to 16
Date: Tue, 18 Mar 2025 16:27:18 -0400
Message-ID: <rdd4m4fze6zt4fjmp2p2ez5smokglno7pqefloyiql36kkudw6@pwpmabadygqz> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<fycuaxeisofdyv4265ejiluixuklmoaaoywa7nhz3heytf4na6@3vd6owgwvadc>
	<[email protected]>

Hi,

On 2025-03-18 16:22:45 -0400, Bruce Momjian wrote:
> On Tue, Mar 18, 2025 at 04:13:26PM -0400, Andres Freund wrote:
> > Hi,
> > 
> > On 2025-03-18 16:08:22 -0400, Bruce Momjian wrote:
> > > This commit makes our default random_page_cost = 4 out of line with
> > > these new settings (assumes modern SSD/NAS/SAN hardware) and more out of
> > > line with reality.
> > 
> > How so? That seems like an independent consideration to me.
> 
> [thread moved to hackers]
> 
> Uh, I think our old random_page_cost and *_io_concurrency assumed
> magnetic disks --- now *_io_concurrency assumes more modern hardware and
> random_page_cost assumes magnetic.

The cost difference between random and non-random IO is actually still
reasonably accurate with NVMEs. You can argue that random_page_cost should be
2.5, but that really depends on the specific hardware.

Particularly for cloud style networked storage, you could even argue that the
difference between sequential and random IO has *grow* given recent changes in
PG (io combining in PG 17), as random IOs much more quickly lead to exhausting
IOPS quotas.

I still don't think adjusting random_page_cost has any meaningful relation to
the change at hand.

Greetings,

Andres Freund

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