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Support direct I/O on macOS.

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Macs don't understand O_DIRECT, but they can disable caching with a
separate fcntl() call.  Extend the file opening functions in fd.c to
handle this for us if the caller passes in PG_O_DIRECT.

For now, this affects only WAL data and even then only if you set:

  max_wal_senders=0
  wal_level=minimal

This is not expected to be very useful on its own, but later proposed
patches will make greater use of direct I/O, and it'll be useful for
testing if developers on Macs can see the effects.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BADiyyHe0cun2wfT%2BSVnFVqNYPxoO6J9zcZkVO7%2BNGig%40mail.gmail.com
This commit is contained in:
Thomas Munro
2021-07-19 08:52:00 +12:00
parent f157db8622
commit 2dbe890571
4 changed files with 83 additions and 19 deletions
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16
src/include/storage/fd.h
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@@ -79,6 +79,22 @@ extern int max_safe_fds;
#define FILE_POSSIBLY_DELETED(err) ((err) == ENOENT || (err) == EACCES)
#endif
/*
* O_DIRECT is not standard, but almost every Unix has it. We translate it
* to the appropriate Windows flag in src/port/open.c. We simulate it with
* fcntl(F_NOCACHE) on macOS inside fd.c's open() wrapper. We use the name
* PG_O_DIRECT rather than defining O_DIRECT in that case (probably not a good
* idea on a Unix).
*/
#if defined(O_DIRECT)
#define PG_O_DIRECT O_DIRECT
#elif defined(F_NOCACHE)
#define PG_O_DIRECT 0x80000000
#define PG_O_DIRECT_USE_F_NOCACHE
#else
#define PG_O_DIRECT 0
#endif
/*
* prototypes for functions in fd.c
*/