Horton

Re: Direct I/O

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@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: 2023-04-14T19:33:39Z
Lists: pgsql-hackers 
Hi,

On 2023-04-14 15:21:18 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2023-04-14 13:21:33 -0400, Tom Lane wrote:
> >> ...  I'm not sure why only those two animals
> >> are unhappy, but I think they have a point: typical ABIs don't
> >> guarantee alignment of function stack frames to better than
> >> 16 bytes or so.  In principle the compiler could support a 4K
> >> alignment request anyway by doing the equivalent of alloca(3),
> >> but I do not think we can count on that to happen.
> 
> > Hm. New-ish compilers seem to be ok with it.
> 
> Oh!  I was misled by the buildfarm label on morepork, which claims
> it's running clang 10.0.1.  But actually, per its configure report,
> it's running
> 
> 	configure: using compiler=gcc (GCC) 4.2.1 20070719 

Huh. I wonder if that was an accident in the BF setup.


> > Perhaps we should have a
> > configure check whether the compiler is OK with that, and disable direct IO
> > support if not?
> 
> +1 for that, though.  (Also, the fact that these animals aren't
> actually failing suggests that 004_io_direct.pl needs expansion.)

It's skipped, due to lack of O_DIRECT:
[20:50:22] t/004_io_direct.pl .............. skipped: no O_DIRECT

So perhaps we don't even need a configure test, just a bit of ifdef'ery? It's
a bit annoying structurally, because the PG*Aligned structs are defined in
c.h, but the different ways of spelling O_DIRECT are dealt with in fd.h.

I wonder if we should try to move those structs to fd.h as well...

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.