Horton

Re: Direct I/O

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andrew Dunstan <andrew@dunslane.net>, Andrea Gelmini <andrea.gelmini@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Noah Misch <noah@leadboat.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, pgsql-hackers@postgresql.org
Date: 2023-04-11T01:55:26Z
Lists: pgsql-hackers 
Hi,

On 2023-04-10 19:27:27 +1200, Thomas Munro wrote:
> On Mon, Apr 10, 2023 at 2:57 PM Andres Freund <andres@anarazel.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
referenced...

Greetings,

Andres Freund

Commits

  1. Rename hook functions for debug_io_direct to match variable name.

  2. Rename io_direct to debug_io_direct.

  3. Skip the 004_io_direct.pl test if a pre-flight check fails.

  4. Use higher wal_level for 004_io_direct.pl.

  5. Skip \password TAP test on old IPC::Run versions

  6. Add io_direct setting (developer-only).

  7. Introduce PG_IO_ALIGN_SIZE and align all I/O buffers.

  8. Add palloc_aligned() to allow aligned memory allocations

  9. initdb: When running CREATE DATABASE, use STRATEGY = WAL_COPY.