Virtual Tape

Contributing Writer, Computerworld |

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With virtual tape, even though we're backing up direct to disk, we pretend we're dealing with tape. Data is backed up to the disk subsystem by accessing it through what's called a virtual tape library—software that emulates the properties of tape. By making the disks look like tape, the virtual system lets IT use its existing tape-based scheduling procedures and practices, scripts and workflows; the only difference is that backup data is stored on a different set of devices. This is such a simplification as to be nearly simple-minded, but it allows IT to expand its capabilities with little or no effort, and it gets away from the need to handle, rotate and store near-line tapes. The net effect is that virtual tape makes both backups and restores faster, more reliable and cheaper.

A Matter of Time

Once upon a time, backups were performed at night, when there were few or no users on the system and there was plenty of time and capacity. Nowadays, users are on systems around the clock, and there's no period when you can shut everything down for backup. As information sources explode and regulation increases, there's so much more to be backed up that we need ever more time and capacity to do so.

So we tell the backup software that it's writing to a tape drive, when in fact we're pointing it at a hard disk. You can think of a disk drive as a large, upfront cache that eliminates delays in changing tape reels or cartridges, or in repositioning tape media for noncontiguous data.

The virtual tape libraries emulate industry-standard-based physical tape drives and libraries, presenting themselves as tape to all of the common backup software applications. A backup media server sends backup streams to a virtual tape library, which writes the data sequentially—that is, in native tape format—to disk storage. Through this bit of hocus-pocus, the virtual tape library appears to the system as anoth-er automated tape library, but the fact that data is being written to disk means backup jobs are completed significantly faster, often by a factor of 10 or more.

Virtual systems emulate tape operations even to the point of assigning bar codes to virtual tape "reels" or "cartridges" used by the backup software.

Virtual tape isn't necessarily the entire answer to backup. It still doesn't address the requirements of off-site storage and disaster recovery, but it can be used with a hierarchical storage management system in which data is moved to slower and increasingly less expensive storage media as it is used less. Virtual tape may also be included as part of a storage-area network, with a single virtual tape server managing less-used or archived data for many networked computers.

A Typical Virtual Tape Configuration

Source: Maxxan Systems Inc., San Jose