Virtual tape libraries in depth


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Symantec's NetBackup OpenStorage (OST) API offers one solution to this problem. With this API, the disk target isn't addressed as a virtual tape or a file system; the backup job is named and passed to the target, and the target stores it however it wants to. Once the backup is stored on the target, NetBackup can tell the IDT to replicate the data; when the replication is done, the IDT tells NetBackup. So, NetBackup is aware of the replicated data and the replication process, and can use it to create a tape copy. The process yields an onsite copy, an offsite disk copy and an offsite tape copy without anyone ever touching a tape. Today, only Data Domain, FalconStor and Quantum Corp. support this API -- and only FalconStor supports it via Fibre Channel; Data Domain and Quantum use IP as their transport.

CommVault Systems Inc. has a similar feature that works with network-attached storage (NAS)-based IDTs (but not VTLs). A media agent watches a directory that you're replicating to and looks for changes. It communicates with the CommServe (the main backup server) and tells it about the other copy, resulting in both copies being available for restores. If this other media agent were located offsite, you could then use that replicated copy to create an offsite tape copy of your replicated backup.

HP also offers this capability for its Data Protector software and the HP Virtual Library System (VLS). The product is similar to CommVault's, except it uses a completely

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separate Data Protector backup server (with its own catalog) to watch for newly replicated virtual tapes. Once those tapes are detected, it asks the other Data Protector server for its catalog information. Both servers can then use those virtual tapes, which would allow creation of a tape copy of the replicated backup.

Software-based VTLs vs. VTL appliances. Because all appliances are just servers running software, the difference between a software VTL and an appliance is more a matter of packaging than a technical issue. It comes down to preferences: prepackaged or build your own. Most VTLs and IDTs are prepackaged, but there are some exceptions, such as the software-only versions of FalconStor's and Gresham's products.

You may also opt to buy a virtual tape library/intelligent disk target with its disk already attached or choose to add your own. In the latter case, options include software-only products or gateway products such as those offered by Data Domain and IBM.

Interoperability with tape libraries. A VTL may provide a direct connection to and integration with a physical tape library. The appeal of this feature has diminished with the increased interest in data deduplication. VTL-tape library integration made it easier to stage data from disk to tape to save space on expensive disk. But with deduplication, there's less need to do this. Products that integrate with physical tape are available from FalconStor, Fujitsu, Gresham, HP and Quantum.

IDTs vs. VTLs. Whether you should back up to a file system device or a virtual tape library truly boils down to personal preference. If you want FC as a transport, your choice is easy; if you want a scalable, deduplicated system, only VTLs offer that today.

File system-based devices have two advantages over virtual tape libraries: what happens when your backup software expires a backup, and simultaneous read and write support.

This was first published in December 2009

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