Enhanced Disk Format (EDF) Architecture

A virtual disk, or minidisk, is a place where you can collect files. Files are what you use to collect logically related data or records. CMS manages the data in files and the files placed on disks using a mapping system. This mapping system is a tree-like structure of pointers and data, where pointers serve as indexes to pieces of data. The amount of pointers and data possible is based on the physical DASD block size of the CMS disk.

CMS disks are formatted into blocks that can be 512, 1KB, 2KB, or 4KB bytes. The block size used is determined when a minidisk, or virtual disk, is formatted. Thus, one disk does not contain a mixture of block sizes. A file consists of data blocks and pointer blocks, which are this same size. The data in a file is broken up into fixed size portions, which are stored on data blocks. Pointer blocks chain the data blocks together. Pointer blocks either point to data blocks or to other pointer blocks.

Choosing an appropriate block size to format a disk depends upon its intended use. A 4KB block size optimizes the I/O if the disk is to contain large files with no missing records (dense). A block size of 1KB is more appropriate when creating many small files or files with missing records (sparse). For example, PL/I regional files are sparse and they may allocate more space on a 4KB disk than on a 1KB disk; therefore, the smaller block size is preferable.

The block size of the disk can affect the amount of storage required for I/O buffers. When caching is not in effect, a larger block size will always result in a greater amount of storage required for the I/O buffers. When caching is in effect, the storage requirements are not determined by the block size of the disk.