FBA Device Geometry
FBA devices appear as linear address spaces of 512-byte blocks. The blocks are consecutively numbered from 0 to n, where n is the highest block number on the volume. z/VM® groups eight consecutive blocks to form a z/VM page and this page must begin on a 4 KB boundary. z/VM then views the volume as a collection of pages numbered from 0 to (n-7)/8. For example, blocks 0-7 make up page 0. There is no concept of cylinder boundaries in this structure.
You must allocate space on FBA volumes in units of pages (in contrast
to units of cylinders on CKD volumes). When you figure your disk space
requirements as a number of pages, you can use these numbers directly
in system configuration file statements or in the ICKDSF service program
(pages). Page numbers must be multiplied by 8 to convert them into
block numbers used by the MDISK directory statement. When assigning
minidisk space, you must know the available extents of your disk in
block numbers (see Table 2).
Note: It is recommended that all FBA minidisks as well as
full-pack minidisk volumes be aligned on page boundaries for optimization
of disk space. Otherwise, the resulting residual blocks of disk space
might not be formatted and utilized. Additional restrictions apply
for SFS file pool minidisks that are not page-aligned. For more information,
see z/VM: CMS File Pool Planning, Administration, and Operation. If the starting block number is a multiple
of eight and the number of blocks is a multiple of eight, then the
minidisk is defined to be on a page boundary.
It is helpful to understand how much disk space you need to allocate for CP storage areas. This chapter shows you how to calculate CP storage areas in terms of the number of cylinders on the various types of CKD devices and the number of 4 KB pages on FBA devices.