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: 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: 2025-03-20T18:54:14Z
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 aio: Fix reference to outdated name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 2025-03-19 18:11:18 -0700, Noah Misch wrote:
> On Wed, Mar 19, 2025 at 06:17:37PM -0400, Andres Freund wrote:
> > On 2025-03-19 14:25:30 -0700, Noah Misch wrote:

> > Hm, we retry more frequently that that if there are new connections...  Maybe
> > just "try again next time"?
> 
> Works for me.
> 

> > > And these individual lines from "git grep BM_IO_IN_PROGRESS":
> > >
> > >  *	i.e at most one BM_IO_IN_PROGRESS bit is set per proc.
> > >
> > > The last especially.
> > 
> > Huh - yea.  This isn't a "new" issue, I think I missed this comment in 16's
> > 12f3867f5534.  I think the comment can just be deleted?
> 
> Hmm, yes, it's orthogonal to $SUBJECT and deletion works fine.
> 
> > > 		 * I/O already in progress.  We already hold BM_IO_IN_PROGRESS for the
> > > 	 * only one process at a time can set the BM_IO_IN_PROGRESS bit.
> > > 	 * only one process at a time can set the BM_IO_IN_PROGRESS bit.
> > 
> > > For the other three lines and the paragraph, the notion
> > > of a process "holding" BM_IO_IN_PROGRESS or being the process to "set" it or
> > > being the process "doing a read" becomes less significant when one process
> > > starts the IO and another completes it.
> > 
> > Hm. I think they'd be ok as-is, but we can probably improve them. Maybe
> 
> Looking again, I agree they're okay.
> 
> > 
> > 	 * Now it's safe to write buffer to disk. Note that no one else should
> > 	 * have been able to write it while we were busy with log flushing because
> > 	 * we got the exclusive right to perform I/O by setting the
> > 	 * BM_IO_IN_PROGRESS bit.
> 
> That's fine too.  Maybe s/perform/stage/ or s/perform/start/.

I put these comment changes into their own patch, as it seemed confusing to
change them as part of one of the already queued commits.


> > > I see this relies on md_readv_complete having converted "result" to blocks.
> > > Was there some win from doing that as opposed to doing the division here?
> > > Division here ("blocks_read = prior_result.result / BLCKSZ") would feel easier
> > > to follow, to me.
> > 
> > It seemed like that would be wrong layering - what if we had an smgr that
> > could store data in a compressed format? The raw read would be of a smaller
> > size. The smgr API deals in BlockNumbers, only the md.c layer should know
> > about bytes.
> 
> I hadn't thought of that.  That's a good reason.

I thought that was better documented, but alas, it wasn't. How about updating
the documentation of smgrstartreadv to the following:

/*
 * smgrstartreadv() -- asynchronous version of smgrreadv()
 *
 * This starts an asynchronous readv IO using the IO handle `ioh`. Other than
 * `ioh` all parameters are the same as smgrreadv().
 *
 * Completion callbacks above smgr will be passed the result as the number of
 * successfully read blocks if the read [partially] succeeds. This maintains
 * the abstraction that smgr operates on the level of blocks, rather than
 * bytes.
 */


I briefly had a bug in test_aio's injection point that lead to *increasing*
the number of bytes successfully read. That triggered an assertion failure in
bufmgr.c, but not closer to the problem.  Is it worth adding an assert against
that to md_readv_complete? Can't quite decide.

Greetings,

Andres Freund