Horton

Re: AIO v2.5

Matthias van de Meent <boekewurm+postgres@gmail.com>

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From: Matthias van de Meent <boekewurm+postgres@gmail.com>
To: Andres Freund <andres@anarazel.de>
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: 2025-07-10T19:00:21Z
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 aio: Fix reference to outdated name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 Wed, 9 Jul 2025 at 16:59, Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2025-07-09 13:26:09 +0200, Matthias van de Meent wrote:
> > I've been going through the new AIO code as an effort to rebase and
> > adapt Neon to PG18. In doing so, I found the following
> > items/curiosities:
> >
> > 1. In aio/README.md, the following code snippet is found:
> >
> > [...]
> > pgaio_io_set_handle_data_32(ioh, (uint32 *) buffer, 1);
> > [...]
> >
> > I believe it would be clearer if it took a reference to the buffer:
> >
> > pgaio_io_set_handle_data_32(ioh, (uint32 *) &buffer, 1);
> >
> > The main reason here is that common practice is to have a `Buffer
> > buffer;` whereas a Buffer * is more commonly plural.
>
> It's also just simply wrong as-is :/. Interpreting the buffer id as a pointer
> obviously makes no sense...

Given that the snippet didn't contain type indications for buffer upto
that point, technically the buffer variable could've been defined as
`Buffer* buffer;` which would've been type-correct. That would be very
confusing however, hence the suggested change.

After your mail, I also noticed the later snippet which should be updated, too:

```
-smgrstartreadv(ioh, operation->smgr, forknum, blkno,
-               BufferGetBlock(buffer), 1);
+void *page = BufferGetBlock(buffer);
+smgrstartreadv(ioh, operation->smgr, forknum, blkno,
+               &page, 1);
```

> > 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?

> > and there is no way for extensions to register the relevant callbacks
> > required to make writev work well on buffered contents. Is that intentional?
>
> FWIW, the problem with writev for buffered IO is not so much with the AIO
> infrastructure, but with buffer locking and sync.c...

Maybe "buffered contents" wasn't the right phrasing, but my main point
remains - I can't seem to find a way to make use of the
pgaio_start_writev API in the 18 branch that doesn't require patching
core code or heavily relying on the behaviour of various pgaio
internals:

The only valid pgaio target is PGAIO_TID_SMGR, which effectively
forces you to use buffered relations as IO target (either shared, or
backend-local/temp). Then, there are no callbacks defined for writev,
so the user will effectively have to do setup and cleanup on their
own, making worker-based IO practically impossible, and requiring the
user to handle all lock acquisition/releases in their own process'
time instead of in the AIO paths - requiring significantly more
involved error handling.


(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)

-----

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]). 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 did notice Linux seems to guarantee it won't read more than
0x7FFF_F000 in any one operation, but I can't find any similar
guarantees for e.g. MacOS or Windows (well, our win32 port limits
pg_pread/writev to 1GB, which seems sufficient for this exact case).

If we're keeping int32, then it would be great if the considerations
for why it's OK to use 32 bits to store the ssize_t vlaue would be
included in the docs/comments.


Kind regards,

Matthias van de Meent
Databricks

[0] https://postgr.es/m/flat/1672202.1703441340%40sss.pgh.pa.us