RE: reads by KTSJ

From: Noveljic Nenad <>
Date: Wed, 13 May 2020 21:12:47 +0000
Message-ID: <>

Thank you Jonathan and Tanel for your invaluable inputs – learned so much from them.

Find my answers below:

  • It isn’t CTAS – last_ddl_time is from long time ago.
  • I haven’t seen any “buffer busy waits”. It was “free buffer waits” instead – block class is not shown with event2 for them.
  • I’ll setup the short stacks with wait events and wait for the next occurrence.

Best regards,


From: <> On Behalf Of Jonathan Lewis Sent: Mittwoch, 13. Mai 2020 21:06
To: ORACLE-L ( <> Subject: Re: reads by KTSJ

This seg fix state is what made me think it might be related to the fix-ups that take place on space allocation from parallel CTAS where every slave creates its own segment and the QC then has to clean-out the blocks that were initially segment header and space management blocks as it combines the separate segments into a single one.

Jonathan Lewis

On Wed, May 13, 2020 at 7:17 PM Tanel Poder <<>> wrote: You mentioned that there were some buffer busy waits showing up too?

Instead of relying on the block address & DBA_EXTENTS query (maybe this block has been reused by something else by now?), you can use ashtop to see which block class the buffer busy waits were against. A random example from my lab:

SQL> _at_ash/dashtop event2 "event='buffer busy waits'" sysdate-90 sysdate

  Seconds AAS %This EVENT2

--------- ------- ------- ---------------------------------------------
    38360      .0   88%   buffer busy waits [data block]
     4680      .0   11%   buffer busy waits [undo data]
      540      .0    1%   buffer busy waits [undo header]
      130      .0    0%   buffer busy waits [file header block]

Also the [ktspsc_segmon] submitted segfix task messages in your tracefiles are interesting.

SQL> SELECT description FROM x$ktsjtaskclass ORDER BY 1;


AIM scheduler
Affinitize segment for IM Hash Index
Segment Fixstate
Segment Monitor

Indeed there's a KTSJ task class called "Segment Fixstate", so maybe Oracle's autonomously fixing something under the hood :)


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Received on Wed May 13 2020 - 23:12:47 CEST

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