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List:       postgresql-general
Subject:    AIX and EAGAIN on open()
From:       Christoph Berg <christoph.berg () credativ ! de>
Date:       2022-06-20 9:53:20
Message-ID: YrBDkPwj5u7HMLCQ () msg ! credativ ! de
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Hello,

a customer running PG on AIX [1] is occasionally seeing "Resource
temporarily unavailable" (EAGAIN) returned by open() calls:

[1] We have PostgreSQL 11.13 on powerpc-ibm-aix7.2.5.0, compiled by \
/opt/IBM/xlc/13.1.0/bin/xlc, 64-bit

2022-05-19 03:28:13 CEST:127.0.0.1(63265):x@x:[64029168]: ERROR:  could not open file \
"base/16401/935915821_fsm": Resource temporarily unavailable 2022-05-19 03:28:13 \
CEST:127.0.0.1(63265):x@x:[64029168]: CONTEXT:  SQL statement "INSERT INTO s[...]"  \
PL/pgSQL function s...() line 12 at SQL statement 2022-05-19 03:28:13 \
CEST:127.0.0.1(63265):x@x:[64029168]: STATEMENT:  PREPARE ... AS insert into ...


2022-04-16 01:45:31 CEST:127.0.0.1(58946):x@x:[20906970]: ERROR:  could not access \
status of transaction 0 2022-04-16 01:45:31 CEST:127.0.0.1(58946):x@x:[20906970]: \
DETAIL:  Could not open file "pg_subtrans/6158": Resource temporarily unavailable. \
2022-04-16 01:45:31 CEST:127.0.0.1(58946):x@x:[20906970]: STATEMENT:  PREPARE ... AS \
update ...


2020-12-01 09:24:30 CET:127.0.0.1(59898):x@x:[6227520]: ERROR:  could not access \
status of transaction 0 2020-12-01 09:24:30 CET:127.0.0.1(59898):x@x:[6227520]: \
DETAIL:  Could not open file "pg_subtrans/AC9E": Resource temporarily unavailable. \
2020-12-01 09:24:30 CET:127.0.0.1(59898):x@x:[6227520]: STATEMENT:  PREPARE ... AS \
DELETE FROM ....


open() should not return EAGAIN as per POSIX [2],

[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html#tag_16_357_05


and the AIX documentation says it would only return EAGAIN if O_TRUNC
is used [3], but as far as I can tell, PG does not use that flag.

[3] https://www.ibm.com/docs/en/aix/7.2?topic=o-open-openat-openx-openxat-open64-open64at-open64x-open64xat-creat-creat64-subroutine


IBM's reply to the issue back in December 2020 was this:

  The man page / infocenter document is not intended as an exhaustive
  list of all possible error codes returned and their circumstances.
  "Resource temporarily unavailable" may also be returned for
  O_NSHARE, O_RSHARE with O_NONBLOCK.

Afaict, PG does not use these flags either.

We also ruled out that the system is using any anti-virus or similar
tooling that would intercept IO traffic.

Does anything of that ring a bell for someone? Is that an AIX bug, a
PG bug, or something else?

Christoph
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