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Three reasons to avoid virtual tape libraries

Virtual tape library (VTL) technology is a subset of disk-to-disk (D2D) backup. Like other forms of D2D, a VTL often uses inexpensive Serial ATA (SATA) drives,

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usually with some level of RAID protection, to provide near-term backup and restoration. Unlike other forms of D2D, a VTL is engineered to look like a very large tape library to the system. Most VTLs can emulate a variety of tape drives and tape libraries.

VTLs can be purchased as software only, as a complete standalone system from vendors such as Copan Systems Inc., Diligent Technologies Corp., FalconStor Software, Network Appliance Inc. (NetApp), Overland Storage Inc. and Sun Microsystems Inc.'s StorageTek, or as a controller for an existing RAID array from companies like Ultera Systems Inc. In addition, some backup software includes a VTL module that uses the existing file system and storage to function as a virtual tape library.

Speed is probably the main reason for installing VTL. Compared to tape, VTLs widen backup windows and make it easier to recover files. VTLs also perform encryption and data deduplication without cramping tight backup windows. While a VTL combines strengths of both tape and disk, it combines some of their weaknesses, too. Storage administrators should consider the drawbacks of VTLs as well as their strengths.

Some reasons not to use a virtual tape library include:

  • A VTL looks just like tape. Most VTL systems are optimized for large block sequential reads and writes, just like tape. VTL operations are much faster than reading and writing a tape, of course, but emulating tape doesn't allow the full performance and efficiency of disk storage.
  • A VTL isn't for archival storage. Despite mimicking tape, VTL is still disk-based. It's good for 30-, 60- or 90-day backups, but it's not designed for long-term preservation of information. Furthermore, unless the VTL is located offsite, it doesn't offer the protection of offsite storage of backups. Most VTL vendors don't recommend trying to use VTL for archival storage. Instead, they suggest creating tapes from the VTL copies. Vendors like NetApp provide features to maintain barcode and indexing continuity from the virtual tape on the VTL to a physical tape.
  • A VTL is a replacement approach to tape backup. The market for VTL is almost entirely existing storage systems. VTL products are designed as replacements or extensions for existing tape-based backup systems. If you're installing a completely new system, straight D2D is a better approach for intermediate backup.

About the author: Rick Cook specializes in writing about issues related to storage and storage management.


This was first published in February 2008

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