Re: Direct I/O

On Wed, Apr 12, 2023 at 3:04 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Wed, Apr 12, 2023 at 2:56 PM Christoph Berg <myon(at)debian(dot)org> wrote:
> > I'm hitting a panic in t_004_io_direct. The build is running on
> > overlayfs on tmpfs/ext4 (upper/lower) which is probably a weird
> > combination but has worked well for building everything over the last
> > decade. On Debian unstable:
> >
> > PANIC: could not open file "pg_wal/000000010000000000000001": Invalid argument

> ... I have a new idea: perhaps it is possible to try
> to open a file with O_DIRECT from perl, and if it fails like that,
> skip the test. Looking into that now.

I think I have that working OK. Any Perl hackers want to comment on
my use of IO::File (copied from examples on the internet that showed
how to use O_DIRECT)? I am not much of a perl hacker but according to
my package manager, IO/File.pm came with perl itself. And the Fcntl
eval trick that I copied from File::stat, and the perl-critic
suppression that requires?

I tested this on OpenBSD, which has no O_DIRECT, so that tests the
first reason to skip.

Does it skip OK on your system, for the second reason? Should we be
more specific about the errno?

As far as I know, the only systems on the build farm that should be
affected by this change are the illumos boxen (they have O_DIRECT,
unlike Solaris, but perl's $^O couldn't tell the difference between
Solaris and illumos, so they didn't previously run this test).

One thing I resisted the urge to do is invent PG_TEST_SKIP, a sort of
anti-PG_TEST_EXTRA. I think I'd rather hear about it if there is a
system out there that passes the pre-flight check, but fails later on,
because we'd better investigate why. That's basically the point of
shipping this "developer only" feature long before serious use of it.