Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>
Date: 2025-03-06T13:13:38Z
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 aio: Fix reference to outdated name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 2025-03-06 12:36:43 +0100, Jakub Wartak wrote:
> On Tue, Mar 4, 2025 at 8:00 PM Andres Freund <andres@anarazel.de> wrote:
> > Questions:
> >
> > - My current thinking is that we'd set io_method = worker initially - so we
> >   actually get some coverage - and then decide whether to switch to
> >   io_method=sync by default for 18 sometime around beta1/2. Does that sound
> >   reasonable?
>
> IMHO, yes, good idea. Anyway final outcomes partially will depend on
> how many other stream-consumers be committed, right?

I think it's more whether we find cases where it performs substantially worse
with the read stream users that exists.  The behaviour for non-read-stream IO
shouldn't change.


> > - To allow io_workers to be PGC_SIGHUP, and to eventually allow to
> >   automatically in/decrease active workers, the max number of workers (32) is
> >   always allocated. That means we use more semaphores than before. I think
> >   that's ok, it's not 1995 anymore.  Alternatively we can add a
> >   "io_workers_max" GUC and probe for it in initdb.
>
> Wouldn't that matter only on *BSDs?

Yea, NetBSD and OpenBSD only, I think.


> > - pg_stat_aios currently has the IO Handle flags as dedicated columns. Not
> >   sure that's great?
> >
> >   They could be an enum array or such too? That'd perhaps be a bit more
> >   extensible? OTOH, we don't currently use enums in the catalogs and arrays
> >   are somewhat annoying to conjure up from C.
>
> s/pg_stat_aios/pg_aios/ ? :^)

Ooops, yes.


> It looks good to me as it is.
> Anyway it
> is a debugging view - perhaps mark it as such in the docs - so there
> is no stable API for that and shouldn't be queried by any software
> anyway.

Cool


> > - Documentation for pg_stat_aios.
>
> pg_aios! :)
>
> So, I've taken aio-2 branch from Your's github repo for a small ride
> on legacy RHEL 8.7 with dm-flakey to inject I/O errors. This is more a
> question: perhaps IO workers should auto-close fd on errors or should
> we use SIGUSR2 for it? The scenario is like this:

When you say "auto-close", you mean that one IO error should trigger *all*
workers to close their FDs?


> so usual stuff with kernel remounting it RO, but here's the dragon
> with io_method=worker:
>
> # mount -o remount,rw /flakey/
> mount: /flakey: cannot remount /dev/mapper/flakey read-write, is
> write-protected.
> # umount /flakey # to fsck or just mount rw again
> umount: /flakey: target is busy.
> # lsof /flakey/
> COMMAND     PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
> postgres 103483 postgres   14u   REG  253,2 36249600   17
> /flakey/tblspace/PG_18_202503031/5/24586
> postgres 103484 postgres    6u   REG  253,2 36249600   17
> /flakey/tblspace/PG_18_202503031/5/24586
> postgres 103485 postgres    6u   REG  253,2 36249600   17
> /flakey/tblspace/PG_18_202503031/5/24586
>
> Those 10348[345] are IO workers, they have still open fds and there's
> no way to close those without restart -- well without close()
> injection probably via gdb.

The same is already true with bgwriter, checkpointer etc?


> pg_terminate_backend() on those won't work. The only thing that works seems
> to be sending SIGUSR2

Sending SIGINT works.


> , but is that safe [there could be some errors after pwrite() ]?

Could you expand on that?


> With
> io_worker=sync just quitting the backend of course works. Not sure
> what your thoughts are because any other bgworker could be having open
> fds there. It's a very minor thing. Otherwise that outage of separate
> tablespace (rarely used) would potentially cause inability to fsck
> there and lower the availability of the DB (due to potential restart
> required).

I think a crash-restart is the only valid thing to get out of a scenario like
that, independent of AIO:

- If there had been any writes we need to perform crash recovery anyway, to
  recreate those writes
- If there just were reads, it's good to restart as well, as otherwise there
  might be pages in the buffer pool that don't exist on disk anymore, due to
  the errors.

Greetings,

Andres Freund