Technical Considerations

VSE/VSAM extends the existing FBA logic to support SCSI disks. VSAM implements a SCSI disk as a generic FBA device and uses its own "virtual characteristics" for mapping and building channel programs for optimized VSAM performance and space utilization.

Except as noted, all commands, parameters, and requirements for FBA devices are valid for SCSI as well.

Several FBA configurations are supported. The generic FBA model is used to:
  • simulate an FBA device in virtual storage; for example, the user can defined in CP: 
    CP DEF VFB-512 AS 152 BLK 100000
    This virtual disk will be presented to the user under VSE as an FBA disk after the initialization with ICKDSF: 
    volume 152                                                                     
AR 0015 CUU  CODE DEV.-TYP   VOLID  USAGE   SHARED    STATUS    CAPACITY       
AR 0015 152  90   9336-10   FBA001  UNUSED                          99960 BLK  
AR 0015 1I40I  READY
  • access a VM minidisk (as part of a real SCSI device). VSAM can address only 2 GB. (z/VM® 5.1 and later releases allows defining VM FBA minidisks with a larger size, but VSE/VSAM can only handle a 2 GB FBA disk in this case.)

    The nature of this 2 GB limit can be explained as follows: VSE/VSAM supports so-called Generic FBA Devices with a virtual FBA disk device characteristic of 64 FBA blocks per track and 15 tracks per cylinder, that is: 960 FBA blocks per cylinder = 4,194,240 bytes per cylinder.

    A space map in the catalog maps each track of a particular disk device with 1 bit (0 = track used, 1 = track free). Additional catalog fields and control blocks map the number of tracks and the start/end track of data spaces and data set extents in 2-byte-fields, which limits the maximum capacity of one DASD device to X'FFFF' = 65,535 tracks (64K - 1).

    Therefore, the current maximum FBA disk capacity is 65,535 tracks * 64 FBA blocks = 4,194,240 FBA blocks = 2,147,450,880 bytes = 2 gigabytes.

  • directly access the SCSI device as an FBA-SCSI device (via FCP). The limit for an FBA-SCSI device is 24 GB, and VSAM can use up to 16 GB on this device by using a different device model (different min-CA and max-CA) as shown in the following table:
    Table 1. Minimum and Maximum CA for Generic FBA Devices
    FBA Device Blocks per Minimum CA Minimum CA per Maximum CA Blocks per Maximum CA
    Generic Virtual FBA 64 15 960
    Generic FBA as VM Minidisk 64 15 960
    FBA-SCSI 512 60 30720
The new device model for SCSI significantly improves the performance (due to fewer CI/CA splits, for example) but requires the system programmers to review, re-calculate, and possibly adapt the space definitions of the JCLs (for example, the minimum size for a catalog (6 min-CAs) and the size for space sub-allocation are different).