CMS disk layout

Both ECKD™ or FBA type DASD can have the CMS disk layout. DASD partitions that are formatted with this layout cannot be accessed by traditional mainframe operating systems. Apart from accessing the disks as ECKD or FBA devices, you can also access them using the DASD DIAG access method.

Figure 1 illustrates two variants of the CMS disk layout.

The first variant contains IPL records, a volume label (CMS1), and a CMS data area. Linux treats DASD like this equivalent to a DASD with the Linux disk layout, where the CMS data area serves as the Linux partition.

The second variant is a CMS reserved volume. In this variant, the DASD was reserved by a CMS RESERVE fn ft fm command. In addition to the IPL records and the volume label, DASD with the CMS disk layout also have CMS metadata. The CMS reserved file serves as the Linux partition.

For both variants of the CMS disk layout, you can have only a single Linux partition. The IPL record, volume label and (where applicable) the CMS metadata, are not intended for use by Linux applications.

Addressing the device and partition is the same for both variants. Linux can address the device as a whole as /dev/dasd<x>, where <x> can be one to four letters that identify the individual DASD. Linux can access the partition as /dev/dasd<x>1.