RE: reads by KTSJ
Date: Wed, 13 May 2020 22:15:31 +0000
Message-ID: <29456_1589408149_5EBC7195_29456_2949_1_00eb22f945734a0a82ee9340219aeef7_at_vontobel.com>
Indeed!
SQL> _at_init
SQL> _at_sys ASSM%fix
NAME VALUE ---------------------------------------------------------------- -------------------------- ASSM bg: segment fix monitor 789 ASSM fg: submit segment fix task 0 ASSM bg:mark segment for fix 0 ASSM bg:create segment fix task 321 ASSM bg:slave fix one segment 169 ASSM bg:slave fix state 8281891
Do you ever run out of good ideas?
Yes, that’s actually my main concern that KTSJs are thrashing the file system.
I’ve already used the hidden parameter, when, e.g. many slaves were started to extend a LOB segment or several slaves tried to do the flashback archive maintenance for the same table, which ended in deadlock. The high default value doesn’t make any sense, especially on a consolidated system. I won’t though change the parameter for now, to get alerted in the case of excessive free buffer waits – it’s a test system anyway.
Best regards,
Nenad
https://nenadnoveljic.com/blog
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Tanel Poder
Sent: Mittwoch, 13. Mai 2020 23:43
To: ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
Subject: Re: reads by KTSJ
If it's lots of single block reads, then the free buffer waits may just show up as a result of high concurrency of reads + block changes (due to whatever perceived need to "fix" the segment blocks). The DBWR wasn't able to keep up with syncing the modified blocks to disk fast enough. The Space management slaves may have inadvertently executed a SLOB-like I/O benchmark workload in your machine :-)
Until you find out if it's segment repair-related and why it kicked in, you could greatly reduce the _max_spacebg_slaves parameter value. It's semi-documented in a couple of MOS notes - in my test lab machine the max number is 1024 (and I have 62 Wnnn processes just waiting around right now):
SQL> _at_pd _max_spacebg_slaves
Show all parameters and session values from x$ksppi/x$ksppcv...
NUM NAME VALUE
---------- -------------------------------------------------------- -----
2604 _max_spacebg_slaves 1024
2605 _minmax_spacebg_slaves 8
That way, even if a repair kicks in, it won't get more than say 8 workers doing all this I/O concurrently.
--
Tanel
http://tanelpoder.com
On Wed, May 13, 2020 at 5:34 PM Tanel Poder <tanel_at_tanelpoder.com<mailto:tanel_at_tanelpoder.com>> wrote: Ok I managed to forget which events you saw as I read through the thread :-)
There are also V$SYSSTAT/V$SESSTAT metrics related to ASSM segment fixing:
SQL> _at_sys ASSM%fix
NAME VALUE ---------------------------------------------------------------- -------------------------- ASSM bg: segment fix monitor 1845 ASSM fg: submit segment fix task 0 ASSM bg:mark segment for fix 0 ASSM bg:create segment fix task 0 ASSM bg:slave fix one segment 0 ASSM bg:slave fix state 0
You could look into AWR and check if you have any unusual numbers there - or run Snapper on the Wnnn processes when the problem happens again.
Also, assuming that the Wnnn slaves haven't exited, you can just check the current V$SESSTAT values (_at_ses2 shows any matching metrics with non-zero values):
SQL> _at_ses2 "select sid from v$session where program like '%(W%'" ASSM%fix
SID NAME VALUE
---------- ---------------------------------------------------------------- ----------
124 ASSM bg: segment fix monitor 40
851 ASSM bg: segment fix monitor 41
1213 ASSM bg: segment fix monitor 41
1578 ASSM bg: segment fix monitor 42
...
Tanel
https://tanelpoder.com
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