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During backups, a virtual tape director behaves like a VTL storing data to its local disk cache. Once cached, however, the data is copied to the appropriate VTL or tape library based on policies set in the backup software. Because the virtual tape director appears like the physical tape library to the software, the virtual tape director can respond to tape library commands issued by the backup software and copy data from disk to tape. This allows the backup software to offload the performance overhead associated with the data movement to the virtual tape director while keeping the backup software's catalog up to date with the creation of physical tape.
Because they virtualize physical tape libraries, virtual tape directors such as the Gresham Clareti VTL may also integrate with physical tape libraries and facilitate faster data recoveries. When the backup software requests data from the Clareti VTL, it will pull the data directly from the disk if it still resides on its disk cache. If the requested data is no longer on disk, the Clareti VTL's integration with tape libraries allows it to recall data from tape faster than using backup software.
When backup software requests data directly from a physical tape library, the software sends the library the information about where the data is positioned on the tape. Because most backup software doesn't know
| how the tape media in the cartridge is physically positioned, the tape drive must rewind the tape to the beginning of the cartridge before it can start to look for the data. However, because the Clareti VTL maintains information about the tape's position in the tape cartridge in its own catalog, it can immediately go to the position on the tape where the data is located without rewinding it.
This was first published in December 2007