parallel recovery slaves waiting on undo reads
Date: Wed, 26 Feb 2020 16:17:27 +0000
Message-ID: <7792_1582733872_5E569A30_7792_5814_1_5412017a5f994eb19f42d4105f041753_at_vontobel.com>
The parallel recovery slaves on a 12.1 physical standby are spending 80% of time in 'db file parallel read':
select program,event,count(*) from v$active_session_history where program like '%PR%' and event='db file parallel read' and sample_time between sysdate-5/24/60 and sysdate group by program,event order by count(*) ;
PROGRAM EVENT COUNT(*) ------------------------------------------------ ---------------------------------------------------------------- ---------- oracle_at_svavt02a (PR04) db file parallel read 202 oracle_at_svavt02a (PR03) db file parallel read 217 oracle_at_svavt02a (PR02) db file parallel read 230 oracle_at_svavt02a (PR01) db file parallel read 237
Most of the reads are on UNDO:
select program,event,t.name, count(*)
from v$active_session_history a, v$datafile f, v$tablespace t
where program like '%PR%' and event='db file parallel read'
and sample_time between sysdate-5/24/60 and sysdate
and a.p1 = f.file# and f.ts#=t.ts#
group by program,event,t.name order by count(*) desc ;
PROGRAM EVENT NAME COUNT(*) ------------------------------------------------ ---------------------------------------------------------------- ------------------------------ ---------- oracle_at_svavt02a (PR02) db file parallel read UNDOTBS1 202 oracle_at_svavt02a (PR01) db file parallel read UNDOTBS1 194 oracle_at_svavt02a (PR04) db file parallel read UNDOTBS1 193 oracle_at_svavt02a (PR03) db file parallel read UNDOTBS1 187 oracle_at_svavt02a (PR01) db file parallel read U1 11 oracle_at_svavt02a (PR04) db file parallel read...
The average wait time strikes out - 18 ms (!):
SQL> select event,total_waits,time_waited,average_wait,max_wait from v$session_event where sid=545 order by time_waited desc ;
EVENT TOTAL_WAITS TIME_WAITED AVERAGE_WAIT MAX_WAIT ---------------------------------------------------------------- ----------- ----------- ------------ ---------- db file parallel read 540946 972298 1.8 553 parallel recovery slave next change 172941 175034 1.01 325 free buffer waits 29682 29615 1 13 write complete waits 85 5005 58.88 548...
However, that's not an IO problem - most of the reads complete in less than 0.5 ms. Below is the distribution of the pread elapsed times in ns:
ns
value ------------- Distribution ------------- count
4096 | 0
8192 | 302
16384 |_at__at_@ 1802
32768 |_at__at_ 1313
65536 | 99
131072 |_at_ 487
262144 |_at__at_@@@@@@@@@ 6761
524288 |_at__at_@@@@@@@@@@@@@ 9270
1048576 |_at__at_@@@@ 3539
2097152 |_at_ 756
4194304 | 45
8388608 | 2
16777216 | 4
33554432 | 0
The output is, by the way, generated by the following script:
#pragma D option quiet
pid$target::pread:entry
{
self->started = timestamp;
/* printf("%d\n",arg0); */
}
pid$target::pread:return
/ self->started /
{
this->duration = timestamp - self->started ;
_at_times["ns"] = quantize(this->duration);
self->started = 0 ;
}
Obviously, 'db file parallel read' measures much more than mere IO or even worse, it might be completely broken.
Any idea what that could be?
I'll switch to async IO tomorrow - maybe its code path is better instrumented.
Any other ideas what to look for?
Thanks in advance,
Nenad
https://nenadnoveljic.com/blog/
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