DB2 FOR LUW / DB2FORLUW / A50 - DB2

LOB values that exceed the inline length are not stored in the 
base row of the table. Instead, they are stored separately in 
the LOB table object. LOB values in the LOB table object are not 
buffered but read by the DB2 engine directly from disk when 
requested. This is called a direct read operation. 
 
Reading and writing larger LOB values from and to disk can be 
time consuming. Therefore the DB2 agent decides based on the 
size of the LOB value if the direct read is performed by the DB2 
agent itself or if it is delegated to the DB2 prefetchers to 
benefit from the ability to read in parallel. 
 
If the LOB read is delegated to the prefetchers the agent has to 
wait for the completion of the prefetch work before it can 
continue. This can lead to a significant wait time if the 
prefetch subsystem is very busy and unable to service the LOB 
prefetch request. 
 
Customers on DB2 10.1 and higher are more likely to experience 
wait times during LOB prefetch during periods of high workload 
because of increased prefetch work from the smart prefetching 
capability. On these releases, NUM_IOSERVER=AUTOMATIC can be too 
low for large systems with high workload when many LOB values 
need to be read or written. 
 
The following changes are effective under DB2_WORKLOAD=SAP: 
1) NUM_IOSERVER=AUTOMATIC is changed to 104 to ensure that 
sufficient prefetchers and prefetch queues are available. 
2) The threshold for the size of a LOB to be prefetched has been 
changed from one extent to the fixed value of 96K. 
 
The formula for the number of prefetchers to start at database 
activation time is changed for users where DB2_WORKLOAD=SAP is 
not set: 
number of prefetchers = 
  max( max( max over all table spaces( parallelism setting ), 
    number of cores * number of SMT threads ), 12 )

**************************************************************** 
* USERS AFFECTED:                                              * 
* ALL                                                          * 
**************************************************************** 
* PROBLEM DESCRIPTION:                                         * 
* See Error Description                                        * 
**************************************************************** 
* RECOMMENDATION:                                              * 
* Upgrade to DB2 Version 10.5 Fix Pack 7.                      * 
****************************************************************

Manually set the database configuration paramater NUM_IOSERVER 
to 104 under DB2_WORKLOAD=SAP.

First fixed in DB2 Version 10.5 Fix Pack 7.

not known / see Local fix

Date  - problem reported    :
Date  - problem closed      :
Date  - last modified       :

09.07.2015
02.10.2015
05.10.2016