Date: Tue, 21 Feb 2023 22:09:37 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Andres Freund <andres@anarazel.de>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, melanieplageman@gmail.com,
	vignesh21@gmail.com, lukas@fittl.com, alvherre@alvh.no-ip.org,
	magnus@hagander.net, pgsql-hackers@postgresql.org,
	thomas.munro@gmail.com, m.sakrejda@gmail.com
Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and
 more?)
Message-ID: <20230222040937.GR1653@telsasoft.com>
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 <20230124223512.xqp7tjgqvzqokaxq@awork3.anarazel.de>
 <20230125.165617.724951472557617342.horikyota.ntt@gmail.com>
 <20230208063814.boyh5ibydbfuo2uk@awork3.anarazel.de>
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On Tue, Feb 21, 2023 at 07:50:35PM -0600, Justin Pryzby wrote:
> On Sat, Feb 11, 2023 at 10:24:37AM -0800, Andres Freund wrote:
> > On 2023-02-08 21:03:19 -0800, Andres Freund wrote:
> > > Pushed the first (and biggest) commit. More tomorrow.
> > 
> > Just pushed the actual pg_stat_io view, the splitting of the tablespace test,
> > and the pg_stat_io tests.
> 
> pg_stat_io says:
> 
>          * Some BackendTypes do not currently perform any IO in certain
>          * IOContexts, and, while it may not be inherently incorrect for them to
>          * do so, excluding those rows from the view makes the view easier to use.
> 
>         if (bktype == B_AUTOVAC_LAUNCHER && io_context == IOCONTEXT_VACUUM)
>                 return false;
> 
>         if ((bktype == B_AUTOVAC_WORKER || bktype == B_AUTOVAC_LAUNCHER) &&
>                 io_context == IOCONTEXT_BULKWRITE)
>                 return false;
> 
> What about these combinations?  Aren't these also "can't happen" ?
> 
>  relation      | bulkread   | autovacuum worker
>  relation      | bulkread   | autovacuum launcher
>  relation      | vacuum     | startup

Nevermind - at least these are possible.

(gdb) p MyBackendType
$1 = B_AUTOVAC_WORKER
(gdb) p io_object
$2 = IOOBJECT_RELATION
(gdb) p io_context
$3 = IOCONTEXT_BULKREAD
(gdb) p io_op
$4 = IOOP_EVICT
(gdb) bt
...
#9  0x0000557b2f6097a3 in ReadBufferExtended (reln=0x7ff5ccee36b8, forkNum=forkNum@entry=MAIN_FORKNUM, blockNum=blockNum@entry=16, mode=mode@entry=RBM_NORMAL, strategy=0x557b305fb568) at ../src/include/utils/rel.h:573
#10 0x0000557b2f3057c0 in heapgetpage (sscan=sscan@entry=0x557b305fb158, block=block@entry=16) at ../src/backend/access/heap/heapam.c:405
#11 0x0000557b2f305d6c in heapgettup_pagemode (scan=scan@entry=0x557b305fb158, dir=dir@entry=ForwardScanDirection, nkeys=0, key=0x0) at ../src/backend/access/heap/heapam.c:885
#12 0x0000557b2f306956 in heap_getnext (sscan=sscan@entry=0x557b305fb158, direction=direction@entry=ForwardScanDirection) at ../src/backend/access/heap/heapam.c:1122
#13 0x0000557b2f59be0c in do_autovacuum () at ../src/backend/postmaster/autovacuum.c:2061
#14 0x0000557b2f59ccf7 in AutoVacWorkerMain (argc=argc@entry=0, argv=argv@entry=0x0) at ../src/backend/postmaster/autovacuum.c:1716
#15 0x0000557b2f59cdd8 in StartAutoVacWorker () at ../src/backend/postmaster/autovacuum.c:1494
#16 0x0000557b2f5a561a in StartAutovacuumWorker () at ../src/backend/postmaster/postmaster.c:5481
#17 0x0000557b2f5a5a39 in process_pm_pmsignal () at ../src/backend/postmaster/postmaster.c:5192
#18 0x0000557b2f5a5d7e in ServerLoop () at ../src/backend/postmaster/postmaster.c:1770
#19 0x0000557b2f5a73da in PostmasterMain (argc=9, argv=<optimized out>) at ../src/backend/postmaster/postmaster.c:1463
#20 0x0000557b2f4dfc39 in main (argc=9, argv=0x557b30568f50) at ../src/backend/main/main.c:200

-- 
Justin