how to find session generating lots of redo



Last updated on August 16th, 2015 at 08:59 am

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.how to find session generating lots of redo
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.

This will need  to executed on all the nodes of database if it is RAC

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.


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