2. USAGE GUIDELINES › 2.2 VTS Component Usage Guidelines › 2.2.2 Virtual Tape Performance Analysis

2.2.2 Virtual Tape Performance Analysis

Virtual tape subsystems are, in large part, self-managed.
The common characteristics of any virtual tape subsystem
are as follows:
 
o A DASD cache (buffer) where virtual tape volumes are
  created and accessed by workloads running on the z/OS host.
 
o Internal software that is similar to traditional tape
  controllers and tape drives so that the z/OS host appears
  to be dealing with real tape devices.
 
o Actual physical tape drives and physical tape cartridges
  that back the DASD cache and achieve high cartridge
  utilization by stacking the virtual volumes created in the
  DASD cache.
 
The DASD cache is a key resource affecting the performance of
all virtual tape systems.  Virtual tape subsystems manage the
DASD cache to meet the following goals:
 
o Ensure there is sufficient free space in the cache to
  accommodate the creation of new virtual tape volumes.
 
o Ensure there is sufficient free space in the cache to
  accommodate the loading of existing volumes from the
  stacked real tape cartridges to satisfy mount requests.
 
o Maximize the cache hit percentage by implementing
  strategies to keep those volumes in cache that are most
  likely to be accessed in the near future.
 
The self-managed nature of virtual tape subsystem cache
requires minimal oversight when the user-specified
installation parameters and virtual tape policies maximize
performance.  If the parameters and policies chosen are not
optimal, however, the system may not perform efficiently.
 
The VTS component database files provide a wealth of
information regarding virtual tape subsystem performance.
Reporting on performance metrics allows a data center to
monitor the impact of different parameter and policy choices
and select those that maximize performance and throughput.
 
Because the subsystem architecture and SMF performance data
for IBM and Sun StorageTek virtual tape systems are
different, the discussion of virtual tape system performance
analysis is divided into the following subsections:
 
    1 - IBM VTS Virtual Tape Performance
    2 - STK VSM Virtual Tape Performance
    3 - IBM TS7700 Hydra Virtual Tape Performance