Re: Direct I/O

On Thu, Dec 22, 2022 at 7:34 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> On Wed, Dec 14, 2022 at 5:48 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> > 0001 -- David's palloc_aligned() patch https://commitfest.postgresql.org/41/3999/
> > 0002 -- I/O-align almost all buffers used for I/O
> > 0003 -- Add the GUCs
> > 0004 -- Throwaway hack to make cfbot turn the GUCs on
>
> David pushed the first as commit 439f6175, so here is a rebase of the
> rest. I also fixed a couple of thinkos in the handling of systems
> where we don't know how to do direct I/O. In one place I had #ifdef
> PG_O_DIRECT, but that's always defined, it's just that it's 0 on
> Solaris and OpenBSD, and the check to reject the GUC wasn't quite
> right on such systems.

Thanks. I have some comments on
v3-0002-Add-io_direct-setting-developer-only.patch:

1. I think we don't need to overwrite the io_direct_string in
check_io_direct so that show_io_direct can be avoided.
2. check_io_direct can leak the flags memory - when io_direct is not
supported or for an invalid list syntax or an invalid option is
specified.

I have addressed my review comments as a delta patch on top of v3-0002
and added it here as v1-0001-Review-comments-io_direct-GUC.txt.

Some comments on the tests added:

1. Is there a way to know if Direct IO for WAL and data has been
picked up programmatically? IOW, can we know if the OS page cache is
bypassed? I know an external extension pgfincore which can help here,
but nothing in the core exists AFAICS.
+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');

2. Can we combine these two append_conf to a single statement?
+$node->append_conf('io_direct', 'data,wal,wal_init');
+$node->append_conf('shared_buffers', '64kB'); # tiny to force I/O

3. A nitpick: Can we split these queries multi-line instead of in a single line?
+$node->safe_psql('postgres', '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;');

4. I don't think we need to stop the node before the test ends, no?
+$node->stop;
+
+done_testing();

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com