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>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-04-14T18:56:32Z
Lists: pgsql-hackers 
Hi,

On 2023-04-14 13:21:33 -0400, Tom Lane wrote:
> Since the direct I/O commit went in, buildfarm animals
> curculio and morepork have been issuing warnings like
> 
> hashpage.c: In function '_hash_expandtable':
> hashpage.c:995: warning: ignoring alignment for stack allocated 'zerobuf'
> 
> in places where there's a local variable of type PGIOAlignedBlock
> or PGAlignedXLogBlock.  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.  Perhaps we should have a
configure check whether the compiler is OK with that, and disable direct IO
support if not?

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.