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From: Nathan Bossart <nathandbossart@gmail.com>
To: Dimitrios Apostolou <jimis@gmx.net>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: [PING] [PATCH v2] parallel pg_restore: avoid disk seeks when jumping short distance forward
Date: Wed, 11 Jun 2025 16:14:53 -0500
Message-ID: <aEnxzX-aPY6nCEvH@nathan> (raw)
In-Reply-To: <c8d94212-72cc-cd9f-162f-8eb845ba104f@gmx.net>
References: <2edb7a57-b225-3b23-a680-62ba90658fec@gmx.net>
	<a7344d8f-927c-1f97-6cc0-ac630deb98c4@gmx.net>
	<c0e531f9-accc-3a93-e9e6-2cec1471f9e9@gmx.net>
	<1282067.1744651649@sss.pgh.pa.us>
	<c6bb93c1-4042-743c-096c-27c5e7c73347@gmx.net>
	<f7a23708-c336-c189-f155-4b3197bf6ea0@gmx.net>
	<aEioDuePEBRnfJYk@nathan>
	<c8d94212-72cc-cd9f-162f-8eb845ba104f@gmx.net>

On Wed, Jun 11, 2025 at 12:32:58AM +0200, Dimitrios Apostolou wrote:
> Thank you for benchmarking! Before answering in more depth, I'm curious,
> what read-seek pattern do you see on the system call level (as shown by
> strace)? In pg_restore it was a constant loop of read(4K)-lseek(8-16K).

For fseeko(), sizes less than 4096 produce a repeating pattern of read()
calls followed by approximately (4096 / size) lseek() calls.  For greater
sizes, it's just a stream of lseek().  For fread(), sizes less than 4096
produce a stream of read(fd, "...", 4096), and for greater sizes, the only
difference is that the last argument is the size.

-- 
nathan





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