Add io_direct setting (developer-only).

Provide a way to ask the kernel to use O_DIRECT (or local equivalent)
where available for data and WAL files, to avoid or minimize kernel
caching.  This hurts performance currently and is not intended for end
users yet.  Later proposed work would introduce our own I/O clustering,
read-ahead, etc to replace the facilities the kernel disables with this
option.

The only user-visible change, if the developer-only GUC is not used, is
that this commit also removes the obscure logic that would activate
O_DIRECT for the WAL when wal_sync_method=open_[data]sync and
wal_level=minimal (which also requires max_wal_senders=0).  Those are
non-default and unlikely settings, and this behavior wasn't (correctly)
documented.  The same effect can be achieved with io_direct=wal.

Author: Thomas Munro <thomas.munro@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGK1X532hYqJ_MzFWt0n1zt8trz980D79WbjwnT-yYLZpg%40mail.gmail.com

2 files changed, 58 insertions, 0 deletions

diff --git a/src/test/modules/test_misc/meson.build b/src/test/modules/test_misc/meson.build
index 21bde427b41..911084ac0fe 100644
--- a/src/test/modules/test_misc/meson.build
+++ b/src/test/modules/test_misc/meson.build
@@ -9,6 +9,7 @@ tests += {
       't/001_constraint_validation.pl',
       't/002_tablespace.pl',
       't/003_check_guc.pl',
+      't/004_io_direct.pl',
     ],
   },
 }
diff --git a/src/test/modules/test_misc/t/004_io_direct.pl b/src/test/modules/test_misc/t/004_io_direct.pl
new file mode 100644
index 00000000000..f5bf0b11e4e
--- /dev/null
+++ b/src/test/modules/test_misc/t/004_io_direct.pl
@@ -0,0 +1,57 @@
+# Very simple exercise of direct I/O GUC.
+
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Systems that we know to have direct I/O support, and whose typical local
+# filesystems support it or at least won't fail with an error.  (illumos should
+# probably be in this list, but perl reports it as solaris.  Solaris should not
+# be in the list because we don't support its way of turning on direct I/O, and
+# even if we did, its version of ZFS rejects it, and OpenBSD just doesn't have
+# it.)
+if (!grep { $^O eq $_ } qw(aix darwin dragonfly freebsd linux MSWin32 netbsd))
+{
+	plan skip_all => "no direct I/O support";
+}
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->append_conf(
+	'postgresql.conf', qq{
+io_direct = 'data,wal,wal_init'
+shared_buffers = '256kB' # tiny to force I/O
+});
+$node->start;
+
+# Do some work that is bound to generate shared and local writes and reads as a
+# simple exercise.
+$node->safe_psql('postgres',
+	'create table t1 as select 1 as i from generate_series(1, 10000)');
+$node->safe_psql('postgres', 'create table t2count (i int)');
+$node->safe_psql(
+	'postgres', qq{
+begin;
+create temporary table t2 as select 1 as i from generate_series(1, 10000);
+update t2 set i = i;
+insert into t2count select count(*) from t2;
+commit;
+});
+$node->safe_psql('postgres', 'update t1 set i = i');
+is( '10000',
+	$node->safe_psql('postgres', 'select count(*) from t1'),
+	"read back from shared");
+is( '10000',
+	$node->safe_psql('postgres', 'select * from t2count'),
+	"read back from local");
+$node->stop('immediate');
+
+$node->start;
+is( '10000',
+	$node->safe_psql('postgres', 'select count(*) from t1'),
+	"read back from shared after crash recovery");
+$node->stop;
+
+done_testing();