Concepts of storage virtualization
Virtualization permits the full utilization of modern tape technology's underlying storage capacity and decreases the virtual volume access time of data storage and retrieval.
virtual volume becomes a physical volume
Virtualization reconfigures the relationship between a physical tape drive, storage media, and the application that reads and writes data. The TS7700 Cache uses disk drive modules (DDMs) as storage media and the TS7700 Server emulates the function and operation of IBM® 3490 Enhanced Capacity (3490E) tape drives. These emulated tape drives are called virtual tape drives, and to an attached host they appear identical to physical tape drives.
Data that is written or read through a virtual tape drive resides in a virtual volume on the DDMs in the TS7700 Cache. To a host application, a virtual volume shows the same characteristics as a physical tape volume. The size of virtual volumes is dependent upon the size of data received from the host; virtual volumes are only as large as necessary to hold the host data. This characteristic of virtual volumes increases the efficiency of layered storage and permits physical tape volumes to maximize available storage.
When the TS7700 is attached to a tape library, the TS7700 Server manages the physical tape drives, or physical volumes, in the tape library and controls the movement of data between physical and virtual volumes. The TS7700 Cache temporarily holds data before writing it to tape, caches the data after a read or write operation to provide fast subsequent access, and provides source data when replicating to another cluster in a Grid. When a virtual volume is read from the TS7700 Cache DDMs and written to a physical volume, premigration is said to have occurred. . Conversely, when a virtual volume is moved from a physical volume to the TS7700 Cache, a recall is said to have occurred . A physical volume can contain one or more virtual volumes; a physical volume that contains more than one virtual volume is referred to as a stacked volume.
When the TS7700 is not attached to a tape library, all data is held in virtual volumes in the TS7700 Cache. In both configurations (with or without an attached physical library), caching algorithms ensure that the cache is full of the most recently created and/or accessed data. The caching algorithms might also be influenced by cache management policies as part of the advanced policy management functions. In both configurations, the replication of a virtual volume from one site to another across the connecting infrastructure (grid network) is referred to as a copy.