Horton

Re: Direct I/O

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Andrew Dunstan <andrew@dunslane.net>, Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>, Christoph Berg <myon@debian.org>, mikael.kjellstrom@gmail.com, pgsql-hackers@postgresql.org
Date: 2023-04-30T23:50:51Z
Lists: pgsql-hackers 
On Sun, Apr 30, 2023 at 06:35:30PM +1200, Thomas Munro wrote:
> On Sun, Apr 30, 2023 at 4:11 PM Noah Misch <noah@leadboat.com> wrote:
> > Speaking of the developer-only status, I find the io_direct name more enticing
> > than force_parallel_mode, which PostgreSQL renamed due to overuse from people
> > expecting non-developer benefits.  Should this have a name starting with
> > debug_?
> 
> Hmm, yeah I think people coming from other databases would be tempted
> by it.  But, unlike the
> please-jam-a-gather-node-on-top-of-the-plan-so-I-can-debug-the-parallel-executor
> switch, I think of this thing more like an experimental feature that
> is just waiting for more features to make it useful.  What about a
> warning message about that at startup if it's on?

Such a warning wouldn't be particularly likely to be seen by someone who
already didn't read/understand the docs for the not-feature that they
turned on.

Since this is -currently- a developer-only feature, it seems reasonable
to rename the GUC to debug_direct_io, and (at such time as it's
considered to be helpful to users) later rename it to direct_io.  
That avoids the issue that random advice to enable direct_io=x under
v17+ is applied by people running v16.  +0.8 to do so.

Maybe in the future, it should be added to GUC_EXPLAIN, too ?

-- 
Justin

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.