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From: Tom Lane <[email protected]>
To: Bruce Momjian <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Cc: Greg Smith <[email protected]>
Subject: Re: Increase default maintenance_io_concurrency to 16
Date: Tue, 18 Mar 2025 17:47:23 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Bruce Momjian <[email protected]> writes:
>>> https://www.postgresql.org/docs/current/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS
>>> 
>>> Random access to mechanical disk storage is normally much more expensive
>>> than four times sequential access. However, a lower default is used
>>> (4.0) because the majority of random accesses to disk, such as indexed
>>> reads, are assumed to be in cache. The default value can be thought of
>>> as modeling random access as 40 times slower than sequential, while
>>> expecting 90% of random reads to be cached.

This explanation is, in fact, made up out of whole cloth.  It has
diddly-squat to do with where the number came from, although maybe
it's an okay rationalization for continuing to use 4.0 on modern
hardware.

Where the number came from is that I did a bunch of testing back in
the late 90s and random_page_cost = 4.0 made the planner's ratios of
seqscan vs indexscan costs match up with observed timings.  Of course,
those observations were made with spinning-rust drives, so you'd not
get the same results on SSDs.

> Yes, I can't say there is much research behind the value, and even if
> there was, the assumed hardware is unlikely to be relevant today.

Yes as to the latter, no as to the former.

			regards, tom lane

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