Advanced Topics › Control and Checkpoint File Considerations › Control File Size Considerations › Change the Size of the Virtual Control File Transfer Buffer

Change the Size of the Virtual Control File Transfer Buffer

The virtual control files use a default transfer buffer of 32,768 bytes to accommodate cross-system transactions.

To change the size of the VCF transfer buffer

1. Specify a value for the VCFBUFFERSIZE parameter on either of the following:
   • The MIMINIT statement on the PARM parameter of the startup procedure
   • The z/OS START command for CA MIM.
2. Specify the same value on all systems.

The lowest possible size is seven bytes more than the block size you specified for the BLKSIZE parameter on the MIMINIT statement. The highest possible size is 62,464 bytes. The value for the VCFBUFFERSIZE parameter should be large enough to accommodate the average number of blocks read per cycle.

Example: VCF transfer buffer size

To increase the size of this buffer to 35,000 bytes, issue the following statement:

MIMINIT VCFBUFFERSIZE=35000

Important! If the buffer size is too small (that is, less than the average number of bytes read per service cycle), CA MIM transmits cross-system transactions too slowly. If you specify too large a buffer size, an excessive amount of real storage is used. To see how many blocks are read per service cycle, issue the following command:

DISPLAY IO

This command also displays other types of I/O statistics for the CA MIM control file.