Re: parallel recovery slaves waiting on undo reads

It’s probably worth seeing if you can catch individual events in v$session_wait_history (shows the last 10 events per session). Importantly, see if the longer durations match up to more requests in the same event.

P1 does not match up to the file#, it is the number of files. I don’t think you can get the individual reads requests from a parallel read from the standard wait interface. There may be an additional event you can set or you might need to use OS tracing to see what files are being accessed.

Hope this helps,
Andy

On Wed, 26 Feb 2020 at 17:18, Noveljic Nenad <nenad.noveljic_at_vontobel.com> wrote:

> 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/
>
>
>
> ____________________________________________________
>
> Please consider the environment before printing this e-mail.
>
> Bitte denken Sie an die Umwelt, bevor Sie dieses E-Mail drucken.
>
>
> Important Notice
> This message is intended only for the individual named. It may contain
> confidential or privileged information. If you are not the named addressee
> you should in particular not disseminate, distribute, modify or copy this
> e-mail. Please notify the sender immediately by e-mail, if you have
> received this message by mistake and delete it from your system.
> Without prejudice to any contractual agreements between you and us which
> shall prevail in any case, we take it as your authorization to correspond
> with you by e-mail if you send us messages by e-mail. However, we reserve
> the right not to execute orders and instructions transmitted by e-mail at
> any time and without further explanation.
> E-mail transmission may not be secure or error-free as information could
> be intercepted, corrupted, lost, destroyed, arrive late or incomplete. Also
> processing of incoming e-mails cannot be guaranteed. All liability of
> Vontobel Holding Ltd. and any of its affiliates (hereinafter collectively
> referred to as "Vontobel Group") for any damages resulting from e-mail use
> is excluded. You are advised that urgent and time sensitive messages should
> not be sent by e-mail and if verification is required please request a
> printed version. Please note that all e-mail communications to and from the
> Vontobel Group are subject to electronic storage and review by Vontobel
> Group. Unless stated to the contrary and without prejudice to any
> contractual agreements between you and Vontobel Group which shall prevail
> in any case, e-mail-communication is for informational purposes only and is
> not intended as an offer or solicitation for the purchase or sale of any
> financial instrument or as an official confirmation of any transaction.
> The legal basis for the processing of your personal data is the legitimate
> interest to develop a commercial relationship with you, as well as your
> consent to forward you commercial communications. You can exercise, at any
> time and under the terms established under current regulation, your rights.
> If you prefer not to receive any further communications, please contact
> your client relationship manager if you are a client of Vontobel Group or
> notify the sender. Please note for an exact reference to the affected group
> entity the corporate e-mail signature. For further information about data
> privacy at Vontobel Group please consult www.vontobel.com.
>

--
http://www.freelists.org/webpage/oracle-l