Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

View thread
From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: 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>, Antonin Houska <ah@cybertec.at>
Date: 2025-03-27T20:58:11Z
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 aio: Fix reference to outdated name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 2025-03-26 21:07:40 -0400, Andres Freund wrote:
> TODO
> ...
> - Add an explicit test for the checksum verification in the completion callback
>
>   There is an existing test for testing an invalid page due to page header
>   verification in test_aio, but not for checksum failures.
>
>   I think it's indirectly covered (e.g. in amcheck), but seems better to test
>   it explicitly.

Ah, for crying out loud.  As it turns out, no, we do not have *ANY* tests for
this for the server-side. Not a single one. I'm somewhat apoplectic,
data_checksums is a really complicated feature, which we just started *turning
on by default*, without a single test of the failure behaviour, when detecting
failures is the one thing the feature is supposed to do.


I now wrote some tests. And I both regret doing so (because it found problems,
which would have been apparent long ago, if the feature had come with *any*
tests, if I had gone the same way I could have just pushed stuff) and am glad
I did (because I dislike pushing broken stuff).

I have to admit, I was tempted to just ignore this issue and just not say
anything about tests for checksum failures anymore.


Problems:

1) PageIsVerifiedExtended() emits a WARNING, just like with ZERO_ON_ERROR, we
   don't want to emit it in a) io workers b) another backend if it completes
   the error.

   This isn't hard to address, we can add PIV_LOG_LOG (or something like that)
   to emit it at a different log level and an out-parameter to trigger sending
   a warning / adjust the warning/error message we already emit once the
   issuer completes the IO.


2) With IO workers (and "foreign completors", in rare cases), the checksum
   failures would be attributed wrongly, as it reports all stats to
   MyDatabaseId

   As it turns out, this is already borked on master for shared relations,
   since pg_stat_database.checksum_failures has existed, see [1].

   This isn't too hard to fix, if we adjust the signature to
   PageIsVerifiedExtended() to pass in the database oid. But see also 3)


3) We can't pgstat_report_checksum_failure() during the completion callback,
   as it *sometimes* allocates memory

   Aside from the allocation-in-critical-section asserts, I think this is
   *extremely* unlikely to actually cause a problem in practice. But we don't
   want to rely on that, obviously.



Addressing the first two is pretty simple and/or needs to be done anyway,
since it's a currently existing bug, as discussed in [1].


Addressing 3) is not at all trivial.  Here's what I've thought of so far:


Approach I)

My first thoughts were around trying to make the relevant pgstat
infrastructure either not need to allocate memory, or handle memory
allocation failures gracefully.

Unfortunately that seems not really viable:

The most successful approach I tried was to report stats directly to the
dshash table, and only report stats if there's already an entry (which
there just about always will, except for a very short period after stats
have been reset).

Unfortunately that fails because to access the shared memory with the stats
data we need to do dsa_get_address(), which can fail if the relevant dsm
segment wasn't already mapped in the current process (it allocates memory
in the process of mapping in the segment). There's no API to do that
without erroring out.

That aspect rules out a number of other approaches that sounded like they
could work - we e.g. could increase the refcount of the relevant pgstat
entry before issuing IO, ensuring that it's around by the time we need to
report. But that wouldn't get around the issue of needing to map in the dsm
segment.


Approach II)

Don't report the error in the completion callback.  The obvious place would be
to do it where we we'll raise the warning/error in the issuing process.  The
big disadvantage is that that that could lead to under-counting checksum
errors:

a) A read stream does 2+ concurrent reads for the same relation, and more than
one encounters checksum errors. When processing the results for the first
failed read, we raise an error and thus won't process the results of the
second+ reads with errors.

b) A read is started asynchronously, but before the backend gets around to
processing the result of the IO, it errors out during that other work
(possibly due to a cancellation). Because the backend never looked at the
results of the IOs, the checksum errors don't get accounted for.

b) doesn't overly bother me, but a) seems problematic.


Approach III)

Accumulate checksum errors in two backend local variables (one for database
specific errors, one for errors on shared relations), which will be flushed by
the backend that issued IO during the next pgstat_report_start().

Two disadvantages:

- Accumulation of errors will be delayed until the next
  pgstat_report_start(). That seems acceptable, after all we do so far a lot
  of other stats.

- We need to register a local callback for shared buffer reads, which don't
  need them today . That's a small bit of added overhead. It's a shame to do
  so for counters that approximately never get incremented.


Approach IV):

Embracy piercing abstractions / generic infrastructure and put two atomic
variables (one for shared one for the backend's database) in some
backend-specific shared memory (e.g. the backend's PgAioBackend or PGPROC) and
update that in the completion callback. Flush that variable to the shared
stats in pgstat_report_start() or such.

This would avoid the need for the local completion callback, and would also
allow to introduce a function see the number "unflushed" checksum errors. It
also doesn't require transporting the number of errors between the shared
callback and the local callback - but we might want to do have that for the
error message anyway.

I wish the new-to-18 pgstat_backend() were designed in a way to make this
possible in a nice way. But unfortunately it puts the backend-specific data in
the dshash table / dynamic shared memory, rather than in a MaxBackends +
NUM_AUX sized array array in plain shared memory. As explained in I), we can't
rely on having the entire array mapped.  Leaving the issue from this email
aside, that also adds a fair bit of overhead to other cases.



Does anybody have better ideas?


I think II), III) and IV) are all relatively simple to implement.

The most complicated bit is that a bit of bit-squeezing is necessary to fit
the number of checksum errors (in addition to the number of otherwise invalid
pages) into the available space for error data. It's doable. We could also
just increase the size of PgAioResult.

I've implemented II), but I'm not sure the disadvantages are acceptable.


Greetings,

Andres Freund

[1] https://postgr.es/m/mglpvvbhighzuwudjxzu4br65qqcxsnyvio3nl4fbog3qknwhg%40e4gt7npsohuz