Re: Show WAL write and fsync stats in pg_stat_io

On Mon, Dec 25, 2023 at 03:20:58PM +0900, Michael Paquier wrote:
> pgstat_tracks_io_bktype() does not look correct to me. Why is the WAL
> receiver considered as something correct in the list of backend types,
> while the intention is to *not* add it to pg_stat_io? I have tried to
> switche to the correct behavior of returning false for a
> B_WAL_RECEIVER, to notice that pg_rewind's test 002_databases.pl
> freezes on its shutdown sequence. Something weird is going on here.
> Could you look at it? See the XXX comment in the attached, which is
> the same behavior as v6-0002. It looks to me that the patch has
> introduced an infinite loop tweaking pgstat_tracks_io_bktype() in an
> incorrect way to avoid the root issue.

Ah, that's because it would trigger an assertion failure:
TRAP: failed Assert("pgstat_tracks_io_op(MyBackendType, io_object,
io_context, io_op)"), File: "pgstat_io.c", Line: 89, PID: 6824
postgres: standby_local: walreceiver
(ExceptionalCondition+0xa8)[0x560d1b4dd38a]

And the backtrace just tells that this is the WAL receiver
initializing a WAL segment:
#5 0x0000560d1b3322c8 in pgstat_count_io_op_n
(io_object=IOOBJECT_WAL, io_context=IOCONTEXT_INIT, io_op=IOOP_WRITE,
cnt=1) at pgstat_io.c:89
#6 0x0000560d1b33254a in pgstat_count_io_op_time
(io_object=IOOBJECT_WAL, io_context=IOCONTEXT_INIT, io_op=IOOP_WRITE,
start_time=..., cnt=1) at pgstat_io.c:181
#7 0x0000560d1ae7f932 in XLogFileInitInternal (logsegno=3, logtli=1,
added=0x7ffd2733c6eb, path=0x7ffd2733c2e0 "pg_wal/00000001", '0'
<repeats 15 times>, "3") at xlog.c:3115
#8 0x0000560d1ae7fc4e in XLogFileInit (logsegno=3, logtli=1) at
xlog.c:3215

Wouldn't it be simpler to just bite the bullet in this case and handle
WAL receivers in the IO tracking?
--
Michael