Horton

Re: AIO v2.5

Noah Misch <noah@leadboat.com>

View thread
From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
Cc: Antonin Houska <ah@cybertec.at>, 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>
Date: 2025-03-25T14:11:20Z
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 aio: Fix reference to outdated name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

On Mon, Mar 24, 2025 at 10:52:19PM -0400, Andres Freund wrote:
> On 2025-03-24 19:20:37 -0700, Noah Misch wrote:
> > On Thu, Mar 20, 2025 at 09:58:37PM -0400, Andres Freund wrote:
> > > +static pg_attribute_always_inline PgAioResult
> > > +buffer_readv_complete_one(uint8 buf_off, Buffer buffer, uint8 flags,
> > > +						  bool failed, bool is_temp)
> > > +{
> > ...
> > > +		if ((flags & READ_BUFFERS_ZERO_ON_ERROR) || zero_damaged_pages)
> > > +		{
> > > +			ereport(WARNING,
> > > +					(errcode(ERRCODE_DATA_CORRUPTED),
> > > +					 errmsg("invalid page in block %u of relation %s; zeroing out page",
> > 
> > My earlier review requested s/LOG/WARNING/, but I wasn't thinking about this
> > in full depth. In the !is_temp case, this runs in a complete_shared
> > callback.  A process unrelated to the original IO may run this callback.
> > That's unfortunate in two ways.  First, that other process's client gets an
> > unexpected WARNING.  The process getting the WARNING may not even have
> > zero_damaged_pages enabled.  Second, the client of the process that staged
> > the IO gets no message.
> 
> Ah, right. That could be why I had flipped it. If so, shame on me for not
> adding a comment...
> 
> 
> > AIO ERROR-level messages handle this optimally.  We emit a LOG-level message
> > in the process that runs the complete_shared callback, and we arrange for the
> > ERROR-level message in the stager.  That would be ideal here: LOG in the
> > complete_shared runner, WARNING in the stager.
> 
> We could obviously downgrade (crossgrade? A LOG is more severe than a LOG in
> some ways, but not others) the message when run in a different backend fairly
> easily.  Still emitting a WARNING in the stager however is a bit more tricky.
> 
> Before thinking more deeply about how we could emit WARNING in the stage:
> 
> Is it actually sane to use WARNING here? At least for ZERO_ON_ERROR that could
> trigger a rather massive flood of messages to the client in a *normal*
> situation. I'm thinking of something like an insert extending a relation some
> time after an immediate restart and encountering a lot of FSM corruption (due
> to its non-crash-safe-ness) during the search for free space and the
> subsequent FSM vacuum.  It might be ok to LOG that, but sending a lot of
> WARNINGs to the client seems not quite right.

Orthogonal to AIO, I do think LOG (or even DEBUG1?) is better for
ZERO_ON_ERROR.  The ZERO_ON_ERROR case also should not use
ERRCODE_DATA_CORRUPTED.  (That errcode shouldn't appear for business as usual.
It should signify wrong or irretrievable query results, essentially.)

For zero_damaged_pages, WARNING seems at least defensible, and
ERRCODE_DATA_CORRUPTED is right.  It wouldn't be the worst thing to change
zero_damaged_pages to LOG and let the complete_shared runner log it, as long
as we release-note that.  It's superuser-only, and the superuser can learn to
check the log.  One typically should use zero_damaged_pages in one session at
a time, so the logs won't be too confusing.

Another thought on complete_shared running on other backends: I wonder if we
should push an ErrorContextCallback that adds "CONTEXT: completing I/O of
other process" or similar, so people wonder less about how "SELECT FROM a" led
to a log message about IO on table "b".

> If we want to implement it, I think we could introduce PGAIO_RS_WARN, which
> then could tell the stager to issue the WARNING. It would add a bit of
> distributed cost, both to callbacks and users of AIO, but it might not be too
> bad.
> 
> 
> > One could simplify things by forcing io_method=sync under ZERO_ON_ERROR ||
> > zero_damaged_pages, perhaps as a short-term approach.
> 
> Yea, that could work.  Perhaps even just for zero_damaged_pages, after
> changing it so that ZERO_ON_ERROR always just LOGs.

Yes.

> Hm, it seems somewhat nasty to have rather different performance
> characteristics when forced to use zero_damaged_pages to recover from a
> problem. Imagine an instance that's configured to use DIO and then needs to
> use zero_damaged_pages to recove from corruption...

True.  I'd be willing to bet high-scale use of zero_damaged_pages is rare.  By
high scale, I mean something like reading a whole large table, as opposed to a
TID scan of the known-problematic range.  That said, people (including me)
expect the emergency tools to be good even if they're used rarely.  You're not
wrong to worry about it.