content_image/973186.jpg
Figure 13-11 shows the TS3500 tape library Shuttle System Architecture with four logical libraries. 

<img src='content_image/973186.jpg'>

## HPSS SCSI PVR interface to the Shuttle Call System 

The HPSS SCSI physical volume repository (PVR) controls a library string of up to eight logical libraries. The HPSS SCSI PVR has at least one connection to, and knowledge of, the contents of each logical library in its configuration. Furthermore, the SCSI PVR is aware of the basic connectivity between logical libraries (HPSS requires all logical libraries to have a direct connection) through the Shuttle Call System (SCS) component, which is an API for managing and controlling shuttle connections. 

When movement across the shuttle is necessary, the HPSS SCSI PVR also can use the SCS component to make an exclusive connection between two shuttle-connected libraries that are available for the move. After it is completed, the SCSI PVR can use SCS to release this exclusive connection and that shuttle connection can serve other requests. 

## Performance considerations 

To move a cartridge from one library string to another, the cartridge is first placed in a shuttle car by the cartridge accessor. The cartridge is then moved across the shuttle connection and is removed by the cartridge accessor at the destination library. The time for the shuttle car to move from one library to the next varies based on installation distances, but is as low as 5 seconds in optimal configurations. 

Mount performance and cartridge move time in a shuttle complex depends on the overall configuration of the complex, including the length of the shuttle connections and the location of the available drive. To maximize performance, the application should use only remote tape drives if all drives are full in the home logical library. 

