RE: reads by KTSJ
Date: Wed, 13 May 2020 09:05:56 +0000
Message-ID: <25726_1589360776_5EBBB888_25726_15767_1_1bc0386b13c946a99b8a3b0c49db7125_at_vontobel.com>
Thank you, Jonathan.
I randomly picked the following block:
SQL> _at_ashtop program2,event2,p1,p1text,p2,p2text,p3,p3text,sql_id,top_level_sql_opcode "program2 like '(W%' and event is not null" "to_date('2020-05-12 13:37:15')" "to_date('2020-05-12 14:37:15')"
It belongs to a table segment:
select owner,segment_name,partition_name,segment_type from dba_extents where file_id=12 and 419748 between block_id and block_id+blocks-1 ;
Below is the the symbolic dump.
Start dump data blocks tsn: 16 file#:12 minblk 419748 maxblk 419748
Block dump from cache:
The block seems empty.
Best regards,
Nenad
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Jonathan Lewis
Sent: Mittwoch, 13. Mai 2020 09:36
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
On Tue, May 12, 2020 at 10:05 PM Noveljic Nenad <nenad.noveljic_at_vontobel.com<mailto: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')"
Some of the affected processes have the following entries in the trace file:
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,
Total Distinct
Seconds AAS %This PROGRAM2 EVENT2 P1 P1TEXT P2 P2TEXT P3 P3TEXT SQL_ID TOP_LEVEL_SQL_OPCODE FIRST_S Execs Seen
--------- ------- ------- ---------------------------------------- ------------------------------------------ ---------- ------------------------------ ---------- ------------------------------ ---------- ------------------------------ ------------- -------------------- ------------- ----------
1 .0 0% | (WnnA) db file sequential read 12 file# 419748 block# 1 blocks 0 2020-05:56:54 1
OWNER SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE
------------------------------ ------------------------------ ------------------------------ ------------------
XXXXXXXX XXXXXXXXXXXXXXXXXXXXXX TABLE
Dump of buffer cache at level 3 for pdb=0 tsn=16 rdba=50751396
Block dump from disk:
buffer tsn: 16 rdba: 0x030667a4 (12/419748)
scn: 0x1211b61c3d seq: 0x01 flg: 0x04 tail: 0x1c3d0601
frmt: 0x02 chkval: 0x51a5 type: 0x06=trans data
Hex dump of block: st=0, typ_found=1
To: ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
Subject: Re: reads by KTSJ
Jonathan Lewis
%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
[ktspsc_segmon] submitted segfix task
[ktspsc_segmon] submitted segfix task
[ktspsc_segmon] submitted segfix task
Nenad
https://nenadnoveljic.com/blog/
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