Cache Management › Static Cache Management › Static Cache Virtual Volume Size

Static Cache Virtual Volume Size

Virtual Volumes are defined during customization as 400 MBs, 800 MBs, or 2000 MBs.

For Static Cache Management, the Virtual Volume Size chosen determines the size of the LDSs that must be defined and initialized on the cache volumes that CA Vtape uses. Each LDS is 1/10th the size of the Virtual Volume Size you choose. One LDS is the smallest unit of DASD allocation for a Virtual Volume. Ten LDSs make up a full Virtual Volume.

400 MB Virtual Volumes are best for sites planning to use CA Vtape for small physical tape data sets to take advantage of automatic stacking. The 40 MB cache LDSs defined to support this Virtual Volume size maximize the number of small tape data sets that can be stored in the cache at one time and minimizes the overall size of the cache. The average uncompressed size of the physical tape data sets should be 400 MBs or less.

800 MB Virtual Volumes are best for sites planning to use CA Vtape for small physical tape data sets to take advantage of automatic stacking and larger physical tape data sets that might fill a physical 3490E tape. The 80 MB cache LDSs defined to support this Virtual Volume size offer a compromise between the most efficient use of the cache DASD space and limiting the increase in the number of Virtual Volumes per data set, which might require the coding of a volume count parameter in the application JCL. A volume count parameter is required whenever more than five tape volumes are required to hold a data set. The average uncompressed size of the physical tape data sets should be less than 800 MBs.

2000 MB Virtual Volumes are best for sites planning to use CA Vtape for physical tape data sets that fill a physical 3490E or larger tape. The 200 MB cache LDSs defined to support this Virtual Volume size provide the maximum relief from the increase in the number of Virtual Volumes that are required to hold these large tape data sets. This minimizes the need to add a volume count parameter to the application JCL.

You can run multiple CA Vtape subsystems on the same LPAR. This allows you to define for example, an 800 MB and a 2000 MB CA Vtape complex on the same LPAR. The CA Vtape filters can then be used to route smaller physical tape data sets to the 800 MB complex and larger physical tape data sets to the 2000 MB complex. While this increases the complexity of the CA Vtape installation and filters, it more efficiently uses the cache DASD space assigned to each complex, maximizes the number of small and large data sets that can reside in the two caches, and minimizes the need to add a volume count parameter to the application JCL.