Using ALIMIT with Virtual Tape Systems

Products

• JMR
• SMR

Issue/Introduction

This Knowledge Document explains the ALIMIT parameter setting when using the virtual tape systems.  

 

Resolution

Virtual tape systems are widely used for archiving and data warehousing applications. The use of this technology has an impact on the archival function in both JMR and SMR.

Both JMR and SMR use the ALIMIT parameter to specify the number of multiple datasets that will reside on a single archive tape.

When using physical tape volumes, optimum efficiency is achieved by stacking multiple ARCHIVE datasets on a single-tape volume. Set the value of the ALIMIT parameter between 10 and 20 files.

When converting to one of the currently available virtual tape systems, the ALIMIT parameter value must be set to 1. Setting this parameter to 1 is critical for two reasons:

1. If the current volume does not contain the number of data sets specified by ALIMIT, another allocation request will be made for that volume. If the virtual tape is in use, the original volume is not in the available virtual tape pool and the mount request cannot be serviced.
2. Stacking datasets on a virtual tape volume is not recommended and will result in lowered efficiency. The virtual tape system initially stores datasets on a mass storage DASD type device until it eventually offloads to physical tape volumes stacked in the order the virtual tape system deems appropriate.