Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

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(at)entry=MAIN_FORKNUM, blockNum=blockNum(at)entry=16, mode=mode(at)entry=RBM_NORMAL, strategy=0x557b305fb568) at ../src/include/utils/rel.h:573
#10 0x0000557b2f3057c0 in heapgetpage (sscan=sscan(at)entry=0x557b305fb158, block=block(at)entry=16) at ../src/backend/access/heap/heapam.c:405
#11 0x0000557b2f305d6c in heapgettup_pagemode (scan=scan(at)entry=0x557b305fb158, dir=dir(at)entry=ForwardScanDirection, nkeys=0, key=0x0) at ../src/backend/access/heap/heapam.c:885
#12 0x0000557b2f306956 in heap_getnext (sscan=sscan(at)entry=0x557b305fb158, direction=direction(at)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(at)entry=0, argv=argv(at)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