Data Lookaside Facility (DLF)

Data Look aside Facility (DLF)

DLF is an alternate fast path method of making Dataset available to many batch jobs faster. Through Hiperbatch, applications can take advantage of the performance benefits of MVS without changing existing application programs or the JCL used to run them.

Hiperbatch mainly depends on DLF to access control over expanded storage.

Initially DLF creats a DLF Object and the user will get connected to DLF inturn user can access through QSAM or VSAM .The DLF object exists as long as no users available to use the data/DLF object.Thus we can have advantage of reduced I/O and faster access to datasets.

Starting DLF :

S DLF,SUB=MSTR,NN=XX

NN=XX                       It indicates which COFDLFxx parmlib member to use .

Stopping DLF :

Before stopping DLF we must enter one of the following Modify command to change its processing to the specified operation.

F DLF,MODE=  {DRAIN|D }

                        {QUIESCE|Q}

 {NORMAL|N }

DRAIN
   In drain mode, DLF connects the user to existing DLF objects or disconnects the user from DLF/

QUIESCE

In quiesce mode, DLF only disconnects the user from DLF objects. No new DLF objects are created while in QUIESCE mode.

NORMAL

In normal mode DLF creates and connects the user to new DLF objects, connects the user to existing DLF objects,and disconnects the user from DLF objects.

Once done with Modify command we can go ahead with STOP command.

P DLF

Display  DLF Status :

F DLF,Status

ParmLib

COFDLFxx

The COFDLFxx parmlib member provides the name of the DLF installation exit and sets limits on the amount of storage that the system can use for Hiperbatch.

COFDLFxx

CLASS     MAXEXPB(megabytes)

          [PCTRETB(percent)]

          [CONEXIT(exit-name)]

MAXEXPB(nnnn)

MAXEXPB is a required parameter that allows you to specify, in megabytes, the maximum amount of storage that the system is to use for Hiperbatch™.

The value for n must be a decimal number from 0 through 9999. You can specify up to 4 digits, including leading zeros. The maximum value you can specify is the maximum amount of storage that is available on your system.

Value Range: 0 - 9999 megabytes

PCTRETB(nnn)

PCTRETB is an optional parameter that allows you to specify the percentage of the storage amount (defined by MAXEXPB) that DLF is to use for retained DLF objects. Retained DLF objects are used when the output of one job step is passed to another job or job step.

The value for n must be a decimal number from 0 through 100. You can specify up to 3 digits, including leading zeros. If the value for n is either not in the range or is not specified, the system uses 0.

Value Range: 0 - 100

Default: 0

CONEXIT(routine-name)

CONEXIT specifies the name of the installation exit routine for DLF. This routine must be reentrant and reside in an authorized library in the LNKLST concatenation. For more information about the DLF installation exit routine.