As the number of Virtual Volumes written increases, a point is reached where little or no unused DASD space exists in the DASD buffer to support new mounts. When this happens, space is automatically freed for reuse.
To determine which space can be safely reused, CA Vtape maintains a queue of all Virtual Volumes that have been Externalized to tape, called the free queue. Virtual Volumes that have not been Externalized are not queued because their LDSs contain the only copy of the application data written.
When an Externalized Virtual Volume is read from the DASD buffer, it is removed from the free queue in case the data is modified, which would require that it be Externalized again. When closed, an unmodified Virtual Volume is requeued at the bottom of the free queue. When closed, a modified Virtual Volume is queued for Externalization. After being Externalized, the Virtual Volume will be queued at the bottom of the free queue.
As a result, Virtual Volumes that are remounted migrate to the bottom of the queue, while Virtual Volumes that are never remounted migrate to the top of the queue. It is these least-recently used Virtual Volumes at the top of the queue that have their space reused to support new virtual tape mounts.
By maintaining the most frequently used Virtual Volumes in the DASD buffer, CA Vtape eliminates the tape mounts required to read the Externalized version of the Virtual Volumes.
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