How to identify the causes of High Redo Generation

admin

(Doc ID 2265722.1)
Purpose Purpose of this document is to show how to identify the causes of excessive redo generation and what we can do to mitigate the problem
Troubleshooting Steps First of all, we need to remark that high redo generation is always a consequence of certain activity in the database and it is expected behavior, oracle is optimized for redo generation and there are no bugs regarding the issue.
The main cause of high redo generation is usually a high DML activity during a certain period of time and it’s a good practice to first examine modifications on either database level (parameters, any maintenance operations,…) and application level (deployment of new application, modification in the code, increase in the users,..).
What we need to examine:
1. Is supplemental logging enabled? The amount of redo generated when supplemental logging is enabled is quite high when compared to when supplemental logging is disabled.
What Causes High Redo When Supplemental Logging is Enabled (Doc ID 1349037.1)
2. Are a lot of indexes being used?, reducing the number of indexes or using the attribute NOLOGGING will reduce the redo considerably
3. Are all the operation really in need of the use of LOGGING? From application we can reduce redo by making use of the clause NOLOGGING. Note that only the following operations can make use of NOLOGGING mode:
- direct load (SQL*Loader)
- direct-load INSERT
- CREATE TABLE ... AS SELECT
- CREATE INDEX
- ALTER TABLE ... MOVE PARTITION
- ALTER TABLE ... SPLIT PARTITION
- ALTER INDEX ... SPLIT PARTITION
- ALTER INDEX ... REBUILD
- ALTER INDEX ... REBUILD PARTITION
- INSERT, UPDATE, and DELETE on LOBs in NOCACHE NOLOGGING mode stored out of line
To confirm if the table or index has "NOLOGGING" set.
Issue the following statement.
select table_name,logging from all_tables where table_name = <table name>;
-or-
select table_name,logging from all_indexes where index_name = <index name>;
4. Do tables have triggers that might cause some indirect DML on other tables?
5. Is Auditing enabled the contributor for this excessive redo generation?
6. Are tablespaces in hot backup mode?
7. Examine the log switches:
select lg.group#,lg.bytes/1024/1024 mb, lg.status, lg.archived,lf.member
from v$logfile lf, v$log lg where lg.group# = lf.group# order by 1, 2;
select to_char(first_time,'YYYY-MON-DD') "Date", to_char(first_time,'DY') day,
to_char(sum(decode(to_char(first_time,'HH24'),'00',1,0)),'999') "00",
to_char(sum(decode(to_char(first_time,'HH24'),'01',1,0)),'999') "01",
to_char(sum(decode(to_char(first_time,'HH24'),'02',1,0)),'999') "02",
to_char(sum(decode(to_char(first_time,'HH24'),'03',1,0)),'999') "03",
to_char(sum(decode(to_char(first_time,'HH24'),'04',1,0)),'999') "04",
to_char(sum(decode(to_char(first_time,'HH24'),'05',1,0)),'999') "05",
to_char(sum(decode(to_char(first_time,'HH24'),'06',1,0)),'999') "06",
to_char(sum(decode(to_char(first_time,'HH24'),'07',1,0)),'999') "07",
to_char(sum(decode(to_char(first_time,'HH24'),'08',1,0)),'999') "08",
to_char(sum(decode(to_char(first_time,'HH24'),'09',1,0)),'999') "09",
to_char(sum(decode(to_char(first_time,'HH24'),'10',1,0)),'999') "10",
to_char(sum(decode(to_char(first_time,'HH24'),'11',1,0)),'999') "11",
to_char(sum(decode(to_char(first_time,'HH24'),'12',1,0)),'999') "12",
to_char(sum(decode(to_char(first_time,'HH24'),'13',1,0)),'999') "13",
to_char(sum(decode(to_char(first_time,'HH24'),'14',1,0)),'999') "14",
to_char(sum(decode(to_char(first_time,'HH24'),'15',1,0)),'999') "15",
to_char(sum(decode(to_char(first_time,'HH24'),'16',1,0)),'999') "16",
to_char(sum(decode(to_char(first_time,'HH24'),'17',1,0)),'999') "17",
to_char(sum(decode(to_char(first_time,'HH24'),'18',1,0)),'999') "18",
to_char(sum(decode(to_char(first_time,'HH24'),'19',1,0)),'999') "19",
to_char(sum(decode(to_char(first_time,'HH24'),'20',1,0)),'999') "20",
to_char(sum(decode(to_char(first_time,'HH24'),'21',1,0)),'999') "21",
to_char(sum(decode(to_char(first_time,'HH24'),'22',1,0)),'999') "22",
to_char(sum(decode(to_char(first_time,'HH24'),'23',1,0)),'999') "23" ,
count(*) Total from v$log_history group by to_char(first_time,'YYYY-MON-DD'), to_char(first_time,'DY')
order by to_date(to_char(first_time,'YYYY-MON-DD'),'YYYY-MON-DD')
This will give us an idea of the times when the high peaks of redo are happening
8. Examine AWR report:
Next step will be examining the AWR from the hour where we have had the highest number of log switches, and confirm with the redo size that these log switches are actually caused by a lot of redo generation.
In the AWR we can also see the sql with most of the gets/executions to have an idea of the activity that is happening in the database and generating redo and we can also see the segments with the biggest number of block changes and the sessions performing these changes.
Another way to find these sessions is described in SQL: How to Find Sessions Generating Lots of Redo or Archive logs (Doc ID 167492.1)
To find these segments we can also use queries:
SELECT to_char(begin_interval_time,'YY-MM-DD HH24') snap_time,
dhso.object_name,
sum(db_block_changes_delta) BLOCK_CHANGED
FROM dba_hist_seg_stat dhss,
dba_hist_seg_stat_obj dhso,
dba_hist_snapshot dhs
WHERE dhs.snap_id = dhss.snap_id
AND dhs.instance_number = dhss.instance_number
AND dhss.obj# = dhso.obj#
AND dhss.dataobj# = dhso.dataobj#
AND begin_interval_time BETWEEN to_date('11-01-28 13:00','YY-MM-DD HH24:MI') <<<<<<<<<<<< Need to modify the time as per the above query where more redo log switch happened (keep it for 1 hour)
AND to_date('11-01-28 14:00','YY-MM-DD HH24:MI') <<<<<<<<<<<< Need to modify the time as per the above query where more redo log switch happened (interval shld be only 1 hour)
GROUP BY to_char(begin_interval_time,'YY-MM-DD HH24'),
dhso.object_name
HAVING sum(db_block_changes_delta) > 0
ORDER BY sum(db_block_changes_delta) desc ;
-- Then : What SQL was causing redo log generation :
SELECT to_char(begin_interval_time,'YYYY_MM_DD HH24') WHEN,
dbms_lob.substr(sql_text,4000,1) SQL,
dhss.instance_number INST_ID,
dhss.sql_id,
executions_delta exec_delta,
rows_processed_delta rows_proc_delta
FROM dba_hist_sqlstat dhss,
dba_hist_snapshot dhs,
dba_hist_sqltext dhst
WHERE upper(dhst.sql_text) LIKE '%<segment_name>%' >>>>>>>>>>>>>>>>>> Update the segment name as per the result of previous query result
AND ltrim(upper(dhst.sql_text)) NOT LIKE 'SELECT%'
AND dhss.snap_id=dhs.snap_id
AND dhss.instance_number=dhs.instance_number
AND dhss.sql_id=dhst.sql_id
AND begin_interval_time BETWEEN to_date('11-01-28 13:00','YY-MM-DD HH24:MI') >>>>>>>>>>>> Update time frame as required
AND to_date('11-01-28 14:00','YY-MM-DD HH24:MI') >>>>>>>>>>>> Update time frame as required
9. Finally, to troubleshoot further the issue and know the exact commands are being recorded at that particular time frame we can use log miner and mine the archivelog from the concerned time frame. We can look on v$archived_log and find the archived log generated at that particular time frame.
How To Determine The Cause Of Lots Of Redo Generation Using LogMiner (Doc ID 300395.1)

• goal: How to find sessions generating lots of redo
• fact: Oracle Server - Enterprise Edition 8
• fact: Oracle Server - Enterprise Edition 9
• fact: Oracle Server - Enterprise Edition 10
  

fix:To find sessions generating lots of redo, you can use either of the following methods. Both methods examine the amount of undo generated. When a transaction generates undo, it will automatically generate redo as well.The methods are:1) Query V$SESS_IO. This view contains the column BLOCK_CHANGES which indicates   how much blocks have been changed by the session. High values indicate a    session generating lots of redo.   The query you can use is:       SQL> SELECT s.sid, s.serial#, s.username, s.program,         2  i.block_changes         3  FROM v$session s, v$sess_io i         4  WHERE s.sid = i.sid         5  ORDER BY 5 desc, 1, 2, 3, 4;   Run the query multiple times and examine the delta between each occurrence   of BLOCK_CHANGES. Large deltas indicate high redo generation by the session.2) Query V$TRANSACTION. This view contains information about the amount of   undo blocks and undo records accessed by the transaction (as found in the    USED_UBLK and USED_UREC columns).   The query you can use is:      SQL> SELECT s.sid, s.serial#, s.username, s.program,         2  t.used_ublk, t.used_urec        3  FROM v$session s, v$transaction t        4  WHERE s.taddr = t.addr        5  ORDER BY 5 desc, 6 desc, 1, 2, 3, 4;   Run the query multiple times and examine the delta between each occurrence   of USED_UBLK and USED_UREC. Large deltas indicate high redo generation by    the session.You use the first query when you need to check for programs generating lots of redo when these programs activate more than one transaction. The latter query can be used to find out which particular transactions are generating redo.


--如何查出redo产生最多的SESSION、SQL

--1、redo大量产生必然是由于大量产生"块改变"。从awr视图中找出"块改变"最多的segments
select * from (
SELECT to_char(begin_interval_time, 'YYYY_MM_DD HH24:MI') snap_time,
       dhsso.object_name,
       SUM(db_block_changes_delta)
  FROM dba_hist_seg_stat     dhss,
       dba_hist_seg_stat_obj dhsso,
       dba_hist_snapshot     dhs
WHERE dhs.snap_id = dhss. snap_id
   AND dhs.instance_number = dhss. instance_number
   AND dhss.obj# = dhsso. obj#
   AND dhss.dataobj# = dhsso.dataobj#
   AND begin_interval_time> sysdate - 60/1440
GROUP BY to_char(begin_interval_time, 'YYYY_MM_DD HH24:MI'),
          dhsso.object_name
order by 3 desc)
where rownum<=5;

1  2014_10_29 08:00  I_FXGL_SWJCDCDXRKXXTJ_C0_SWGLM  5374160
2  2014_10_29 08:00  LS_CB_CJJLMX  1987024
3  2014_10_29 08:00  PK_SK_QSXX_C0_ND  1981648
4  2014_10_29 08:00  SK_QSXX_C0_ND  1949696
5  2014_10_29 08:00  I_136830960702392855_PZ_XH_YZ  1421952




--2、从awr视图中找出步骤1中排序靠前的对象涉及到的SQL
SELECT to_char(begin_interval_time, 'YYYY_MM_DD HH24:MI'),
       dbms_lob.substr(sql_text, 4000, 1),
       dhss.instance_number,
       dhss.sql_id,
       executions_delta,
       rows_processed_delta
  FROM dba_hist_sqlstat dhss, dba_hist_snapshot dhs, dba_hist_sqltext dhst
WHERE UPPER(dhst.sql_text) LIKE '%这里写对象名大写%'
   AND dhss.snap_id = dhs.snap_id
   AND dhss.instance_Number = dhs.instance_number
   AND dhss.sql_id = dhst.sql_id;
   
--3、从ASH相关视图中找出执行这些SQL的session、module、machine
select * from v$active_session_history where sql_Id = 'c1hn9qb2nkm7r'


-------------------------------------------------------------------------------------------------
--回滚段征用
SELECT A.sid,A.serial#  serial_num,D.NAME     RNAME,C.UBABLK,B.SQL_TEXT,A.username user_name,A.osuser   os_user_name,A.terminal terminal,
A.program  program FROM V$SESSION A, V$SQLAREA B, V$TRANSACTION C, v$rollname D
WHERE A.TADDR = C.ADDR AND A.SQL_ADDRESS = B.ADDRESS AND A.SQL_HASH_VALUE = B.HASH_VALUE AND C.xidusn = D.usn;