Reading ahead tracks for sequential file access

You can use a Data Virtualization Manager Server Event Facility (SEF) rule to specify the number of tracks to read ahead (MULTACC) when reading sequential data sets for individual requests.

About this task

Using a virtual table (VTB) rule, you can specify the number of tracks to read ahead (the MULTACC parameter value) for MapReduce sequential file access for individual requests. This support overrides the value in the server parameter ACIMAPREDUCETRACKS (NUMBER OF MAP REDUCE TRACKS TO READ) for individual requests. Sample VTB rule AVZMLTAC is provided.

To override the MULTACC value, you must enable the AVZMLTAC rule and use the MACC_nn_ prefix for table names in your SQL statement.

The AVZMLTAC rule is invoked every time a table with the prefix MACC_nn_ is found in the SQL statement. The following format is expected: 
MACC_nn_virtualtablename
Where:
  • nn is the number of tracks to read ahead (the MULTACC value) when reading sequential data sets
  • virtualtablename is the name of the virtual table
For example: 
SELECT * from MACC_15_STAFF_SSEQ ;
The following message is displayed in the Server Trace: 
AVZ1000I VTB.OPTBMACC set to 15

Use the following procedure to set up the rule.

Procedure

  1. Customize the Data Virtualization Manager configuration member (AVZSIN00) to enable virtual table rule events by configuring the SEFVTBEVENTS parameter in the member, as follows: 
    "MODIFY PARM NAME(SEFVTBEVENTS) VALUE(YES)"
  2. Access the VTB rules, as follows:
    1. In the Data Virtualization Manager server - Primary Option Menu, specify option E, Rules Mgmt.
    2. Specify option 2, SEF Rule Management.
    3. Enter VTB for Display Only the Ruleset Named.
  3. Enable the rule by specifying E next to AVZMLTAC and pressing Enter.
  4. Set the rule to Auto-enable by specifying A next to AVZMLTAC and pressing Enter.

    Setting a rule to Auto-enable activates the rule automatically when the server is re-started.