Virtual tape libraries in depth


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For disk-based backup, VTLs have been a relatively easy way to replace traditional tape libraries. With added features such as deduplication, they can be an attractive alternative to other disk target systems.

Virtual tape libraries (VTLs) are dead, right? Weren't they supposed to be temporary solutions that would be long forgotten once everyone started backing up to "real" disk? While that might be what the VTL naysayers had in mind, we're more than a few years into the VTL "fad" and many of the products are doing just fine.

What happened was that an industry segment morphed to encompass both VTLs and intelligent disk targets (IDTs), a segment that was ultimately validated when EMC Corp. acquired Data Domain for $2.4 billion. We'll review some of the factors that led to the development of VTLs, the current state of VTL technologies and products (including the newer features they now offer), and then we'll end with a look into the future of VTLs and IDTs.

Why VTLs came about

The VTL/IDT market has become so overshadowed by the data deduplication craze that some people may have forgotten why the industry developed VTLs in the first place.

Tape was (and is) too fast. The core problem vendors were trying to solve with virtual tape libraries is the mismatch between the speed of tape and the speed of the disk drives, file systems and databases they're backing up. In approximately 15 years, the sustained throughput

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of open system disk drives has gone from approximately 4 MBps to 70 MBps -- an increase of 1,700%. In roughly the same amount of time, the sustained throughput of open system tape drives has grown from 256 KBps (Exabyte EXB-8200 drive) to 180 MBps (LTO-4) -- an increase of 70,000%.

VTLs made the unfamiliar familiar. For many backup administrators and their backup software, backing up to disk was a foreign concept. Knowing that progress in backup systems is an incremental process, VTL vendors felt they could take the unfamiliar (disk) and make it seem like an old friend (tape).

Scalable. Demand for VTLs has been driven by the needs of large enterprise customers. With a tape library, they had hundreds or thousands of tapes and dozens or hundreds of tape drives, and they could just throw all their backups at this big tape library and it would sort it out. To use disk, however, they would need to manage and load balance their backups across dozens to hundreds of discreet disk systems.

The VTL solved this problem by presenting disk as large tape libraries, something they were already familiar with. In various ways, VTL vendors made dozens of individual disk arrays look like one or more tape libraries that could scale to almost limitless levels.

Shareable. Because backup software already knew how to share tape libraries, they could easily share VTLs. Instead of using extra-cost sharing software (such as Symantec Corp.'s Veritas NetBackup Shared Storage Option or EMC Corp.'s NetWorker Dynamic Drive Sharing Option), you could create as many "tape drives" as you needed to give each backup server its own tape drives, while dynamically sharing the VTL. And if you have multiple backup applications that refuse to share, a VTL can be carved into separate virtual libraries.

Fragmentation issues with file system devices. VTLs also avoid the fragmentation issues associated with backing up to file systems. They solved this problem using proprietary file systems that wrote data contiguously.

This was first published in December 2009

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