Advanced Topics › Control and Checkpoint File Considerations › How CA MIM Formats Control and Checkpoint Files

How CA MIM Formats Control and Checkpoint Files

The CA MIM control and checkpoint files are allocated without specifying RECFM, LRECL, BLKSIZE, or DSORG attributes. This is intentional, because CA MIM does not use standard access methods (for example, VSAM or QSAM) to perform I/O operations to these files. Instead, it provides a special formatting routine, which prepares the file for access by CA MIM channel programs. CA MIM builds its own channel programs to perform I/O operations to DASD and checkpoint files. The same is true for CTC I/O operations. CA MIM uses Cross System Extended Services (XES) to access coupling facility structure control files.

In addition to structuring the files, the formatting logic creates a special record called the Global Management Record (GMR). The GMR is built primarily on DEFSYS, GLOBALVALUE, and MIMINIT values and describes the systems and recovery options that define the MIMplex. The basic components of this record are created only during the formatting process.

The GMR resides in the DASD control file or coupling facility structure control file when you use the DASDONLY or CTCDASD communication methods. The GMR resides in the checkpoint file when you use the CTCONLY or XCF communication methods.

A FORMAT startup is required under the following circumstances:

• You are planning to reference new, unformatted DASD or coupling facility structure, or checkpoint files for the first time
• You are planning to make changes to the DEFSYS statement
• You are planning to make changes to the MIMINIT statement following parameters:
  • BLKSIZE
  • COMMUNICATION
  • GCMF
  • GDIF
  • GTAF
  • VCFMAXBLOCKS