Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>, Antonin Houska <ah@cybertec.at>
Date: 2025-03-29T03:35:23Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. aio: Fix assertion, clarify README

  2. aio: Fix reference to outdated name

  3. aio: Fix possible state confusions due to interrupt processing

  4. aio: Improve debug logging around waiting for IOs

  5. aio: Fix crash potential for pg_aios views due to late state update

  6. Increase BAS_BULKREAD based on effective_io_concurrency

  7. localbuf: Add Valgrind buffer access instrumentation

  8. aio: Make AIO more compatible with valgrind

  9. aio: Avoid spurious coverity warning

  10. tests: Fix incompatibility of test_aio with *_FORCE_RELEASE

  11. tests: Cope with WARNINGs during failed CREATE DB on windows

  12. aio: Add errcontext for processing I/Os for another backend

  13. aio: Add README.md explaining higher level design

  14. aio: Minor comment improvements

  15. aio: Add test_aio module

  16. aio: Add pg_aios view

  17. docs: Add acronym and glossary entries for I/O and AIO

  18. Enable IO concurrency on all systems

  19. read_stream: Introduce and use optional batchmode support

  20. docs: Reframe track_io_timing related docs as wait time

  21. bufmgr: Use AIO in StartReadBuffers()

  22. bufmgr: Implement AIO read support

  23. aio: Add WARNING result status

  24. Let caller of PageIsVerified() control ignore_checksum_failure

  25. pgstat: Allow checksum errors to be reported in critical sections

  26. Add errhint_internal()

  27. localbuf: Track pincount in BufferDesc as well

  28. aio, bufmgr: Comment fixes/improvements

  29. Fix mis-attribution of checksum failure stats to the wrong database

  30. aio: Implement support for reads in smgr/md/fd

  31. aio: Add io_method=io_uring

  32. aio: Add liburing dependency

  33. aio: Rename pgaio_io_prep_* to pgaio_io_start_*

  34. aio: Pass result of local callbacks to ->report_return

  35. aio: Be more paranoid about interrupts

  36. Redefine max_files_per_process to control additionally opened files

  37. aio: Change prefix of PgAioResultStatus values to PGAIO_RS_

  38. bufmgr: Improve stats when a buffer is read in concurrently

  39. aio: Add io_method=worker

  40. aio: Infrastructure for io_method=worker

  41. aio: Add core asynchronous I/O infrastructure

  42. aio: Basic subsystem initialization

  43. tests: Expand temp table tests to some pin related matters

  44. localbuf: Introduce FlushLocalBuffer()

  45. localbuf: Introduce TerminateLocalBufferIO()

  46. localbuf: Fix dangerous coding pattern in GetLocalVictimBuffer()

  47. localbuf: Introduce StartLocalBufferIO()

  48. localbuf: Introduce InvalidateLocalBuffer()

  49. Allow lwlocks to be disowned

  50. Make jsonb casts to scalar types translate JSON null to SQL NULL.

  51. bufmgr/smgr: Don't cross segment boundaries in StartReadBuffers()

  52. Use aux process resource owner in walsender

  53. bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

Hi,

On 2025-03-28 08:54:42 -0400, Andres Freund wrote:
> On 2025-03-27 20:22:23 -0700, Noah Misch wrote:
> > On Thu, Mar 27, 2025 at 04:58:11PM -0400, Andres Freund wrote:
> > > - We need to register a local callback for shared buffer reads, which don't
> > >   need them today . That's a small bit of added overhead. It's a shame to do
> > >   so for counters that approximately never get incremented.
> >
> > Fair concern.  An idea is to let the complete_shared callback change the
> > callback list associated with the IO, so it could change
> > PGAIO_HCB_SHARED_BUFFER_READV to PGAIO_HCB_SHARED_BUFFER_READV_SLOW.  The
> > latter would differ from the former only in having the extra local callback.
> > Could that help?  I think the only overhead is using more PGAIO_HCB numbers.
>
> I think changing the callback could work - I'll do some measurements in a
> coffee or two, but I suspect the overhead is not worth being too worried about
> for now.  There's a different aspect that worries me slightly more, see
> further down.
> ...
> Unfortunately pgstat_prepare_report_checksum_failure() has to do a lookup in a
> local hashtable. That's more expensive than an indirect function call
> (i.e. the added local callback). I hope^Wsuspect it'll still be fine, and if
> not we can apply a mini-cache for the current database, which is surely the
> only thing that ever matters for performance.

I tried it and at ~30GB/s of read IO, with checksums disabled, I can't see a
difference of either having the unnecessary complete_local callback or having
the lookup in pgstat_prepare_report_checksum_failure(). In a profile there are
a few hits inside pgstat_get_entry_ref(), but not enough to matter.

Hence I think this isn't worth worrying about, at least for now. I think we
have far bigger fish to fry at this point than such a small performance
difference.

I've adjusted the comment above TRACE_POSTGRESQL_BUFFER_READ_DONE() to not
mention the overhead. I'm still inclined to think that it's better to call it
in the shared completion callback.


I also fixed support and added tests for ignore_checksum_failure, that also
needs to be determined at the start of the IO, not in the completion.  Once
more there were no tests, of course.


I spent the last 6 hours on the stupid error/warning messages around this,
somewhat ridiculous.

The number of combinations is annoyingly large. It's e.g. plausible to use
ignore_checksum_failure=on and zero_damaged_pages=on at the same time for
recovery. The same buffer could both be ignored *and* zeroed. Or somebody
could use ignore_checksum_failure=on but then still encounter a page that is
invalid.

But I finally got to a point where the code ends up readable, without undue
duplication.  It would, leaving some nasty hack aside, require a
errhint_internal() - but I can't imagine a reason against introducing that,
given we have it for the errmsg and errhint.

Here's the relevant code:

	/*
	 * Treat a read that had both zeroed buffers *and* ignored checksums as a
	 * special case, it's too irregular to be emitted the same way as the other
	 * cases.
	 */
	if (zeroed_any && ignored_any)
	{
		Assert(zeroed_any && ignored_any);
		Assert(nblocks > 1);	/* same block can't be both zeroed and ignored */
		Assert(result.status != PGAIO_RS_ERROR);
		affected_count = zeroed_or_error_count;

		ereport(elevel,
				errcode(ERRCODE_DATA_CORRUPTED),
				errmsg("zeroing %u pages and ignoring %u checksum failures among blocks %u..%u of relation %s",
					   affected_count, checkfail_count, first, last, rpath.str),
				affected_count > 1 ?
				errdetail("Block %u held first zeroed page.",
						  first + first_off) : 0,
				errhint("See server log for details about the other %u invalid blocks.",
						affected_count + checkfail_count - 1));
		return;
	}

	/*
	 * The other messages are highly repetitive. To avoid duplicating a long
	 * and complicated ereport(), gather the translated format strings
	 * separately and then do one common ereport.
	 */
	if (result.status == PGAIO_RS_ERROR)
	{
		Assert(!zeroed_any);	/* can't have invalid pages when zeroing them */
		affected_count = zeroed_or_error_count;
		msg_one = _("invalid page in block %u of relation %s");
		msg_mult = _("%u invalid pages among blocks %u..%u of relation %s");
		det_mult = _("Block %u held first invalid page.");
		hint_mult = _("See server log for the other %u invalid blocks.");
	}
	else if (zeroed_any && !ignored_any)
	{
		affected_count = zeroed_or_error_count;
		msg_one = _("invalid page in block %u of relation %s; zeroing out page");
		msg_mult = _("zeroing out %u invalid pages among blocks %u..%u of relation %s");
		det_mult = _("Block %u held first zeroed page.");
		hint_mult = _("See server log for the other %u zeroed blocks.");
	}
	else if (!zeroed_any && ignored_any)
	{
		affected_count = checkfail_count;
		msg_one = _("ignoring checksum failure in block %u of relation %s");
		msg_mult = _("ignoring %u checksum failures among blocks %u..%u of relation %s");
		det_mult = _("Block %u held first ignored page.");
		hint_mult = _("See server log for the other %u ignored blocks.");
	}
	else
		pg_unreachable();

	ereport(elevel,
			errcode(ERRCODE_DATA_CORRUPTED),
			affected_count == 1 ?
			errmsg_internal(msg_one, first + first_off, rpath.str) :
			errmsg_internal(msg_mult, affected_count, first, last, rpath.str),
			affected_count > 1 ? errdetail_internal(det_mult, first + first_off) : 0,
			affected_count > 1 ? errhint_internal(hint_mult, affected_count - 1) : 0);

Does that approach make sense?

What do you think about using
  "zeroing invalid page in block %u of relation %s"
instead of
  "invalid page in block %u of relation %s; zeroing out page"

I thought about instead translating "ignoring", "ignored", "zeroing",
"zeroed", etc separately, but I have doubts about how well that would actually
translate.

Greetings,

Andres Freund