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Don Burleson Blog 







Insufficient ITL Slots Tips

Expert Oracle Database Tips by Donald BurlesonMarch 22, 2015

Detecting Insufficient ITL Slots

Oracle keeps track of rows that are locked per transaction in a storage area at the top of each data block called the Interested Transaction List (ITL). Two parameters, initrans and maxtrans, control the number of ITL slots in any block for an object within Oracle.

Iitrans contains the total number of slots initially created in an Oracle database block when it is first used. The maxtrans parameter sets the upper bound for the total number of entries allowed within the interested transaction list for the block. As such, each transaction that plans to modify a database block will require a new slot in the ITL list contained within the block. The maxtrans parameter also sets the upper limit for the number of concurrent transactions that may be active at any single point in time within an Oracle database block.

The initrans parameter also grants the minimum concurrency set per block. If the value is greater for initrans but less than the values available for maxtrans for transactions that are active concurrently within the same block, then the ITL list will be further extended. However, space must be available within the database block for the ITL list to be expanded.

If there are no more free ITL lists available, then the requesting session will wait on one of the active transaction locks to complete its transaction. How ITL works with database locking can be investigated by using the following example:

Session 1:

SQL> update txns set txt='Garbage' where num=1;

   1 row updated.

Session 2:

SQL> update lab.txns set txt='Junkers' where num=2;

   1 row updated.

Now examine the locks held, if any:

SQL> select sid, type, id1, id2, lmode, request
  2  from v$lock
  3  where type='TX';

       SID TY        ID1        ID2      LMODE    REQUEST
---------- -- ---------- ---------- ---------- ----------
       137 TX     589844        821          6          0
       134 TX     196613        705          6          0
       138 TX     393217        702          6          0

If there was a lock issue, the REQUEST column would display a value and the correlated SID column would be the session waiting for the lock.

One can check the ITL waits by querying the v$segment_statistics dynamic performance view as shown in the following example.

SQL> desc v$segment_statistics

    Name                                      Null?    Type
    ----------------------------------------- -------- --------------- 
    OWNER                                              VARCHAR2(30)
    OBJECT_NAME                                        VARCHAR2(30)
    SUBOBJECT_NAME                                     VARCHAR2(30)
    TABLESPACE_NAME                                    VARCHAR2(30)
    TS#                                                NUMBER
    OBJ#                                               NUMBER
    DATAOBJ#                                           NUMBER
    OBJECT_TYPE                                        VARCHAR2(18)
    STATISTIC_NAME                                     VARCHAR2(64)
    STATISTIC#                                         NUMBER
    VALUE                                              NUMBER


   v$segment_statistics t
   t.statistic_name='ITL waits'

If there were any ITL related waits, the above query would return the details.


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