Horton

Re: Direct I/O

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, Andrew Dunstan <andrew@dunslane.net>, 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: 2023-04-09T23:46:16Z
Lists: pgsql-hackers 
On Mon, Apr 10, 2023 at 8:43 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Boy, it's hard to look at that trace and not call it a filesystem bug.

Agreed.

> Given the apparent dependency on COW, I wonder if this has something
> to do with getting confused about which copy is current?

Yeah, I suppose it would require bogus old page versions (or I guess
alternatively completely mixed up page offsets) rather than bogus
zeroed pages to explain the too-high count observed in one of crake's
failed runs: I guess it counted some pre-updated tuples that were
supposed to be deleted and then counted the post-updated tuples on
later pages (insert joke about the Easter variant of the Halloween
problem).  It's just that in the runs I've managed to observe and
analyse, the previous version always happened to be zeros.

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.