Horton

Re: Direct I/O

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Greg Stark <stark@mit.edu>
Cc: Thomas Munro <thomas.munro@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Andrew Dunstan <andrew@dunslane.net>, Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>, Christoph Berg <myon@debian.org>, mikael.kjellstrom@gmail.com, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-04-19T17:13:55Z
Lists: pgsql-hackers 
Hi,

On 2023-04-18 15:35:09 -0400, Greg Stark wrote:
> On Mon, 17 Apr 2023 at 17:45, Thomas Munro <thomas.munro@gmail.com> wrote:
> >  It's something you'd
> > opt into for a dedicated database server along with other carefully
> > considered settings.  It seems acceptable to me that if you set
> > io_direct to a non-default setting on an unusual-for-a-database-server
> > filesystem you might get errors screaming about inability to open
> > files -- you'll just have to turn it back off again if it doesn't work
> > for you.
> 
> If the only solution is to turn it off perhaps the server should just
> turn it off? I guess the problem is that the shared_buffers might be
> set assuming it would be on?

I am quite strongly opposed to that - silently (or with a log message, which
practically is the same as silently) disabling performance relevant options
like DIO is much more likely to cause problems, due to the drastically
different performance characteristics you get. I can see us making it
configurable to try using DIO though, but I am not convinced it's worth
bothering with that. But we'll see.

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.