Re: Direct I/O

From: Andres Freund <andres(at)anarazel(dot)de>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Andrea Gelmini <andrea(dot)gelmini(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Noah Misch <noah(at)leadboat(dot)com>, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Direct I/O
Date: 2023-04-11 01:55:26
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On 2023-04-10 19:27:27 +1200, Thomas Munro wrote:
> On Mon, Apr 10, 2023 at 2:57 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Have you tried to write a reproducer for this that doesn't involve postgres?
> I tried a bit. I'll try harder soon.
> > ... What kernel version did you repro
> > this on Thomas?
> Debian's 6.0.10-2 kernel (Debian 12 on a random laptop). Here's how I
> set up a test btrfs in case someone else wants a head start:
> truncate -s2G 2GB.img
> sudo losetup --show --find 2GB.img
> sudo mkfs -t btrfs /dev/loop0 # the device name shown by losetup
> sudo mkdir /mnt/tmp
> sudo mount /dev/loop0 /mnt/tmp
> sudo chown $(whoami) /mnt/tmp
> cd /mnt/tmp
> /path/to/initdb -D pgdata
> ... (see instructions further up for postgres command line + queries to run)

I initially failed to repro the issue with these instructions. Turns out that
the problem does not happen if huge pages are in used - I'd configured huge
pages, so the default huge_pages=try succeeded. As soon as I disable
huge_pages explicitly, it reproduces.

Another interesting bit is that if checksums are enabled, I also can't
reproduce the issue. Together with the huge_page issue, it does suggest that
this is somehow related to page faults. Which fits with the thread Andrea


Andres Freund

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