Overview of z/VM Support for SCSI Devices
z/VM® supports SCSI FCP disk logical units (SCSI disks) for both system and guest use. SCSI disks can be used directly by a guest operating system when an FCP subchannel is dedicated to a guest. Such a guest must contain its own SCSI device driver – Linux® on IBM Z® is one such guest.
SCSI disks can also be used as emulated 9336 model 20 fixed-block-architecture (FBA) disks. CMS and CP rely almost exclusively on this emulated-FBA support for their SCSI usage. Specifically, this usage includes system paging, spooling, directory services, minidisks, and all other system functions and programming services that support FBA disks. Guests that support FBA disks (such as CMS, GCS, RSCS, Linux, and VSE) also can use SCSI disks through the emulated-FBA support, without requiring any specific SCSI support in the guests.
z/VM supports emulated FBA disks and their underlying SCSI disks up to 1 terabyte minus 1 page (2,147,483,640 512-byte blocks) in size, without regard to the capacity of real 9336 disks.
When CP uses FBA disks, space is allocated by 4096-byte pages. FBA DASD space allocated for use as directory, paging, and spooling must reside within the first 16,777,215 pages (64 GB minus 1 page) of the volume. Other types of CP allocations (TDSK, PERM, and PARM) can exist beyond the first 64 GB. CMS minidisks have absolute and practical capacity limitations. See the notes in CMS Restrictions.
- FCP device number
- A real device number for a subchannel associated with an FCP channel, providing access to the fibre-channel fabric.
- Target worldwide port name (WWPN)
- The unique worldwide port name associated with a target port on a SCSI controller.
- Logical unit number (LUN)
- The number of a specific logical unit (i.e. logical device) associated with the target port.
Refer to Figure 1 for an illustration of these required elements.
Figure 2 illustrates the overall system architecture related to z/VM SCSI support. The figure shows how channel programs for emulated FBA disks are processed through an emulation layer and a SCSI driver into the fibre-channel fabric. The figure also shows how guests with their own SCSI support can use a dedicated FCP subchannel to access the fabric directly. Note that the elements described above in Figure 1 are contained in the FCP Channel and Storage Area Network portions of Figure 2.
