Tuning the VTAM system
As you begin testing with the DB2® distributed data facility, you probably need to modify VTAM® options and CDB values to handle certain potential problems.
About this task
You should consult a VTAM communications expert to tune your network. Consider the following tasks when tuning VTAM for DDF.
- Controlling buffer storage
By sending large amounts of data through the network, DB2 can cause problems with your VTAM I/O buffer pool.
- Controlling pacing
You probably need to tune your pacing options if your VTAM buffers become overloaded with data that is sent to this local DB2.
- Modifying default session limits
Consider modifying session limits if you have problems obtaining enough sessions to handle your distributed workload efficiently.
- Modifying class of service
Specifying a class of service can help you assign priorities to your network applications.
- Associating applications with modes
Tuning the system can require that you add new modes to your log mode table so that there is a greater variety of classes of service available for your sessions. This variety allows you to have more flexibility in tuning the system for specific uses. This topic tells you how to associate specific sessions with modes.
Before you begin tuning the network, you must understand the relationship between VTAM options and associated values in the CDB for DB2. The table below summarizes the relationship.
| Macro Name | Option | CDB table.column | Relationship |
|---|---|---|---|
| APPL | name | LOCATIONS.LINKNAME LUNAMES.LUNAME LUMODES.LUNAME MODESELECT.LUNAME USERNAMES.LINKNAME LULIST.LINKNAME | The LU name used in VTAM communication. This name maps 1:1 to the system's location name in LOCATIONS. |
| APPL | DSESLIM | LUMODES.CONVLIMIT | CONVLIMIT overrides session limits specified with DSESLIM. Session limit values are used in CNOS processing. |
| MODEENT | LOGMODE | LUMODES.MODENAME | LUMODES creates session limits for specific LU name and mode name combinations. |
| MODEENT | LOGMODE | MODESELECT.MODENAME | MODESELECT maps authorization IDs and plans to specific modes. |
- Controlling buffer storage
VTAM uses buffer pools for control blocks, network traffic data, and channel programs. A shortage of buffer pools can have an adverse effect on VTAM CPU time, storage consumption, and the ability to serve DB2 requests. - Session-level pacing
Session-level pacing is the mechanism by which the receiver of data (DB2, in this case) can control the pace at which the sender sends data (in the form of RUs). - Modifying class of service
You can define the transmission priority and paths between systems with entries in a class of service (COS) table. - Associating applications with modes
As you tune your system, you can assign certain applications, such as a high-priority job, to the mode that is best suited for that job. - When updates to CDB values take effect for VTAM connections
Any table in the CDB can be updated while DDF is active. - Calculating session limits
You can calculate session
limit values for DRDA access.
- Calculating VTAM I/O buffer pool (IOBUF) storage
You can estimate VTAM buffer pool storage when you use the distributed data facility of DB2. Every path information unit (PIU) that enters or leaves VTAM resides in one or more IOBUF buffers. - CNOS processing in DB2
The distributed data facility of DB2 can request to alter the number of sessions with another system for a specific VTAM logon mode. This automatic process is called change number of sessions (CNOS).