RE: reads by KTSJ

From: Noveljic Nenad <>
Date: Wed, 13 May 2020 22:15:31 +0000
Message-ID: <>


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,


From: <> On Behalf Of Tanel Poder Sent: Mittwoch, 13. Mai 2020 23:43
To: ORACLE-L ( <> 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.


On Wed, May 13, 2020 at 5:34 PM Tanel Poder <<>> 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


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Received on Thu May 14 2020 - 00:15:31 CEST

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