Re: reads by KTSJ

From: Jonathan Lewis <jlewisoracle_at_gmail.com>
Date: Wed, 13 May 2020 08:35:32 +0100
Message-ID: <CAGtsp8=o3j9PfQq9MnwU-uSibagh8EnNNERznHOUiy2LZMk7zw_at_mail.gmail.com>



As Mladen said, KTS is all about space management, and one of the key functions of the Wnnn processes is to anticipate space requirements and pre-allocate, which applies particularly to extending tablespaces in advance of demand and extending segments (particularly securefile segments, I think) in advance.

You said the buffer busy waits were in the user schema - which presumably means they were for blocks that are currently part of some user segments. If this is correct, what class of block would they be - I'd expect them to be bitmap space management blocks. I can imagine Oracle using several Wnnn processes to add a large amount of space to a securefile segment and having to update the L3 and L2 bitmap blocks for that segment - and having to queue to update the bitmaps.

Regards
Jonathan Lewis

On Tue, May 12, 2020 at 10:05 PM Noveljic Nenad <nenad.noveljic_at_vontobel.com> wrote:

> I started occasionally seeing KTSJ slaves heavily waiting on “free buffer
> waits” and “db file sequential read” after upgrading from 18.7.0.0.190716
> to 19.7.0.0.200414:
>
>
>
> SQL> _at_ash_wait_chains2 "program2||' '||event2||' '||module||' '||action"
> "program2 like '(W%'" "to_date('2020-05-12 13:37:15')"
> "to_date('2020-05-12 14:37:15')"
>
>
>
> -- Display ASH Wait Chain Signatures script v0.5 BETA by Tanel Poder (
> http://blog.tanelpoder.com )
>
>
>
> %This SECONDS AAS WAIT_CHAIN
> FIRST_SEEN
> LAST_SEEN
>
> ------ ---------- -------
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ------------------- -------------------
>
> 22% 310 .1 -> (Wnnn) free buffer waits KTSJ KTSJ Slave ->
> (DBWnnn) db file parallel write
> 2020-05-12
> 13:46:55 2020-05-12 13:48:40
>
> 10% 144 .0 -> (Wnnn) db file sequential read KTSJ KTSJ
> Slave
> 2020-05-12
> 13:46:54 2020-05-12 13:48:52
>
> 10% 144 .0 -> (WnnA) free buffer waits KTSJ KTSJ Slave ->
> (DBWnnn) db file parallel write
> 2020-05-12
> 13:46:55 2020-05-12 13:57:13
>
> 7% 104 .0 -> (WnnA) db file sequential read KTSJ KTSJ
> Slave
> 2020-05-12 13:46:54 2020-05-12
> 13:57:11
>
>
>
> Some of the affected processes have the following entries in the trace
> file:
>
>
>
> *** SESSION ID:(1202.8580) 2020-05-12T13:46:54.295884+02:00
>
> *** CLIENT ID:() 2020-05-12T13:46:54.295916+02:00
>
> *** SERVICE NAME:(SYS$BACKGROUND) 2020-05-12T13:46:54.295947+02:00
>
> *** MODULE NAME:(KTSJ) 2020-05-12T13:46:54.295978+02:00
>
> *** ACTION NAME:(KTSJ Slave) 2020-05-12T13:46:54.296009+02:00
>
> *** CLIENT DRIVER:() 2020-05-12T13:46:54.296039+02:00
>
>
>
> [ktspsc_segmon] submitted segfix task
>
> [ktspsc_segmon] submitted segfix task
>
> [ktspsc_segmon] submitted segfix task
>
>
>
>
>
> The tablespaces are autoextensible, but I don’t know if they were extended
> during this time period. The IO performance is OK.
>
>
>
> I examined one of the read bloccks – it belongs to a user schema, nothing
> special about it:
>
>
>
> How to figure out what might have caused this KTSJ slave activity?
>
>
>
> Best regards,
>
>
>
> Nenad
>
>
>
> https://nenadnoveljic.com/blog/
>
>
>
>
>
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Received on Wed May 13 2020 - 09:35:32 CEST

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