Horton

Re: AIO v2.5

Aleksander Alekseev <aleksander@timescale.com>

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From: Aleksander Alekseev <aleksander@timescale.com>
To: pgsql-hackers@postgresql.org
Cc: Andres Freund <andres@anarazel.de>, Noah Misch <noah@leadboat.com>, 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: 2025-04-01T11:56:17Z
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 aio: Fix reference to outdated name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 Andres,

> > I didn't yet push
> >
> > > > Subject: [PATCH v2.14 13/29] aio: Add README.md explaining higher level design

I have several notes about 0003 / README.md:

1. I noticed that the use of "Postgres" and "postgres" is inconsistent.

2.

```
+pgaio_io_register_callbacks(ioh, PGAIO_HCB_SHARED_BUFFER_READV, 0);
```

Perhaps I'm a bit late here, but the name of the function is weird. It
registers a single callback, but the name is "_callbacks".

3. The use of "AIO Handle" and "AioHandle" is inconsistent.

4.

- pgaio_io_register_callbacks
- pgaio_io_set_handle_data_32

If I understand correctly one can register multiple callbacks per one
AIO Handle (right? ...). However I don't see an obvious way to match
handle data to the given callback. If all the callbacks get the same
handle data... well it's weird IMO, but we should explicitly say that.
On top of that we should probably explain in which order the callbacks
are going to be executed. If there are any guarantees in this respect
of course.

5. pgaio_io_set_handle_data_32(ioh, (uint32 *) buffer, 1)

Perhaps it's worth mentioning if `buffer` can be freed after the call
i.e. if it's stored by value or by reference. It's also worth
clarifying if the maximum number of buffers is limited or not.

6. It is worth clarifying if AIO allows reads and writes or only reads
at the moment. Perhaps it's also worth explicitly saying that AIO is
for disk IO only, not for network one.

7. It is worth clarifying how many times the callbacks are called when
reading multiple buffers. Is it guaranteed that the callbacks are
called ones, or if it somehow depends on the implementation, and also
what happens in the case if I/O succeeds partially.

8. I believe we should tell a bit more about the context in which the
callbacks are called. Particularly what happens to the memory contexts
and if I can allocate/free memory, can I throw ERRORs, can I create
new AIO Handles, is it expected that the callback should return
quickly, are the signals masked while the callback is executed, can I
use sockets, is it guaranteed that the callback is going to be called
in the same process (I guess so, but the text doesn't explicitly
promise that), etc.

9.

```
+Because acquisition of an IO handle
+[must always succeed](#io-can-be-started-in-critical-sections)
+and the number of AIO Handles
+[has to be limited](#state-for-aio-needs-to-live-in-shared-memory)
+AIO handles can be reused as soon as they have completed.
```

What pgaio_io_acquire() does if we are out of AIO Handles? Since it
always succeeds I guess it should block the caller in this case, but
IMO we should say this explicitly.

10.

> > because I want to integrate some language that could be referenced by
> > smgrstartreadv() (and more in the future), as we have been talking about.
>
> I tried a bunch of variations and none of them seemed great. So I ended up
> with a lightly polished version of your suggested comment above
> smgrstartreadv().  We can later see about generalizing it.

IMO the problem here is that README doesn't show the code that does IO
per se, and thus doesn't give the full picture of how AIO should be
used. Perhaps instead of referencing smgrstartreadv() it would be
better to provide a simple but complete example, one that opens a
binary file and reads 512 bytes from it by the given offset for
instance.

-- 
Best regards,
Aleksander Alekseev