Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Antonin Houska <ah@cybertec.at>, Noah Misch <noah@leadboat.com>, 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: 2025-07-10T19:29:33Z
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 aio: Fix reference to outdated name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 2025-07-10 21:00:21 +0200, Matthias van de Meent wrote:
> On Wed, 9 Jul 2025 at 16:59, Andres Freund <andres@anarazel.de> wrote:
> > > 3. I noticed that there is AIO code for writev-related operations
> > > (specifically, pgaio_io_start_writev is exposed, as is
> > > PGAIO_OP_WRITEV), but no practical way to excercise that code: it's
> > > not called from anywhere in the project, and there is no way for
> > > extensions to register the relevant callbacks required to make writev
> > > work well on buffered contents. Is that intentional?
> >
> > Yes.  We obviously do want to support writes eventually, and it didn't seem
> > useful to not have the most basic code for writes in the AIO infrastructure.
> >
> > You could still use it to e.g. write out temporary file data or such.
>
> Yes, though IIUC that would require an implementation of at least
> PgAioTargetInfo for such a use case (it's definitely not a SMGR
> target), which currently isn't available and can't be registered
> dynamically by an extension. Or maybe did I miss something?

I can see some hacky ways around that, but they're just that, hacky...



> (PS. I'm not quite 100% sure that it is impossible to use, just that
> there are rather few handles available for using this part of the new
> tool, and it seems completely untested in the PG18 branch)

I'm not saying it's 100% ready to use without modifying core code, but for
something that's like 30 lines of code, as part of a considerably larger
subystem, I just don't see a problem with writev not yet being covered.  It's
just incremental development.


> -----
>
> Something else I've just noticed is the use of int32 in
> PgAIOHandle->result. In sync and worker mode, pg_preadv and pg_pwritev
> return ssize_t, which most modern systems can't fit in int32 (the
> output was int before, then size_t, then ssize_t: [0]).

I don't think there's anything that can actually do IO that's large enough to
be problematic. What's the potential scenario where you'd want to read/write
more than 3GB of data within one syscall? That just doesn't seem to make
sense.


> While not directly an issue in default PG18 due to the use of 1GB relation
> segments capping the max IO size for SMGR-managed IOs (and various other
> code-level constraints), this may have more issues when an extension starts
> bulk-reading data on a system compiled with RELSEG_SIZE >= 2GB; I can't find
> any protective checks against overflows in downcasting the IO result.

I don't think the relation size is relevant piece here, it's just that it
doesn't make sense (and likely isn't possible) to read that much data at once.


Greetings,

Andres Freund