Horton

Re: Direct I/O

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Andrew Dunstan <andrew@dunslane.net>, Andres Freund <andres@anarazel.de>, 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: 2023-04-09T02:05:05Z
Lists: pgsql-hackers 
Thomas Munro <thomas.munro@gmail.com> writes:
> It's odd, though, if it is their bug and not ours: I'd expect our
> friends in other databases to have hit all that sort of thing years
> ago, since many comparable systems have a direct I/O knob*.

Yeah, it seems moderately likely that it's our own bug ... but this
code's all file-system-ignorant, so how?  Maybe we are breaking some
POSIX rule that btrfs exploits but others don't?

> I gather that btrfs is actually Fedora's default file system (or maybe
> it's just "laptops and desktops"[2]?).

I have a ton of Fedora images laying about, and I doubt that any of them
use btrfs, mainly because that's not the default in the "server spin"
which is what I usually install from.  It's hard to guess about the
buildfarm, but it wouldn't surprise me that most of them are on xfs.
(If we haven't figured this out pretty shortly, I'm probably going to
put together a btrfs-on-bare-metal machine to try to duplicate crake's
results.)

			regards, tom lane

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.