2. USAGE GUIDELINES › 2.2 VTS Component Usage Guidelines › 2.2.3 Virtual Tape Capacity Planning

2.2.3 Virtual Tape Capacity Planning

Capacity planning for virtual tape systems primarily involves
understanding the current workload, the trending of key
utilization and performance metrics, and understanding future
workload requirements.  Understanding the current workload
can be accomplished by trending certain key metrics,
available from both the IBM VTS and StorageTek VSM systems:

o Average numbers of virtual and physical mounts, by category

  Understanding the daily patterns and long-term patterns of
  virtual volume access is critical in making decisions about
  workload shifting and activating new workloads that use
  virtual tape.


o Average virtual tape mount time

  Increases in the average virtual tape mount time can
  indicate the need for additional DASD cache, an examination
  of policies that govern which virtual volumes stay in cache
  longer relative to others, insufficient physical drives,
  and other reasons.

o Cache misses

  Virtual tape requests that cannot be satisfied in the
  DASD cache involve the recall of volumes from a physical
  cartridge.  This seriously prolongs the time required
  to satisfy the z/OS host workload request.

o Average virtual volume size

  Virtual tape is generally not suitable for extremely large
  data sets.  Too many excessively large data sets can
  dominate the DASD cache.  Average virtual volume size
  should be monitored over time to ensure that users of
  virtual tape are using the facility appropriately.