Overview of the z/VSE Support for SCSI Disks

Small Computer System Interface (SCSI) disks are widely used in open systems. SCSI disks that support the Fibre Channel Protocol (FCP) can be attached to IBM® servers, as well. The objective of the z/VSE® SCSI support is to offer clients more storage choices. Starting with z/VSE 3.1, z/VSE supports FCP-attached SCSI disks in addition to FBA and (E)CKD disks. SCSI disk devices use fixed block 512-bytes sectors, and therefore SCSI I/O commands are block oriented. The same is true for Fixed Block Architecture (FBA) disks. FBA disks are organized in fixed block sectors of 512 bytes. A record can consist of multiple blocks and is addressed by its relative block number. I/O programming is done using FBA channel programs. Since both SCSI and FBA devices have an underlying block structure, z/VSE SCSI is implemented using the z/VSE FBA support. Few configuration commands are needed to define and work with SCSI disks. Once configured, SCSI disks are seen to the operator, administrator and programmer as FBA disks and can be used with existing FBA interfaces. FBA I/O channel commands are translated internally into SCSI I/O commands. This is done at low level I/O interfaces and therefore transparent to user and system and vendor programs.

User, system, and vendor programs run unchanged provided they are:
  • device-independent or
  • use FBA channel programs.
z/VSE SCSI provides
  • SCSI disks can be used both in an LPAR and a z/VM® guest environment.
  • z/VSE can be installed and IPLed from SCSI. Therefore it is possible to build a 'SCSI-only' z/VSE system.
  • This includes DASD sharing with the lock file on SCSI.
  • It is possible to mix and match by, for example, having your z/VSE system on ECKD and using SCSI disks in addition for selected user data.
  • Stand-alone dump to a SCSI disk.
  • SCSI disk size
    • z/VSE supports SCSI disks from 8 MB to 24 GB. Because z/VSE itself uses the first 4 MB of a SCSI disk for internal purposes, the available user space is equal to the defined size of the disk minus 4 MB.
    • VSE/VSAM files must be placed in the first 16 GB of a SCSI disk.
  • Multi-Pathing, which is a method to provide high availability of a device.
  • Point-to-Point connections (DS8000®, DS6000™ only)
  • N_Port ID Virtualization (NPIV)

z/VSE FCP-attached SCSI disk support complements SCSI support in z/VM and Linux® on z Systems. Using the z/VM emulated FBA support, z/VM presents SCSI disks as 9336-20 FBA disks. For z/VSE these are real FBA disks, not SCSI disks.