For Joel Larkin, network specialist with the Pension Benefits Guarantee Corp. (PBGC)--a government agency in Washington, DC that pays benefits to workers whose pension plans have closed--disk was the answer to decreasing the time it took to do a weekly full backup. Equipped with an ADIC Scalar 458 library with DLT 7000 drives (10MB/s compressed throughput or 36GB/hr), it was taking PBGC the entire weekend to backup approximately 3TB. With thousands of tapes to use up before being able to justify a drive upgrade, Larkin set out to find a way to "get our backup window down from all weekend to under a day."

PBGC bought three Quantum DX30s, 3TB IDE arrays that emulate a P1000 tape library. Rated at about 80MB/s, (288GB/hr), PBGC should be able to get their backup done in well under a day--"provided our servers can keep up," Larkin says.

But can disk speed up all backups? "The answer, like everything in enterprise storage, is, 'It depends,'" says Dave Kenyon, product line manager for enhanced backup at Quantum, which begins shipping the 10TB DX100 disk-based backup accelerator this fall.

According to Kenyon, if tape is the bottleneck, then yes, putting a disk cache between the backup server and the tape can speed up your backup. But if clients or the backup server are the bottleneck, disk won't do much to improve the speed of your backup. It will, however, protect your drives and media from the dreaded "shoeshine effect," where tapes that aren't receiving data fast enough have to stop, back up and start again.

Nevertheless, disk-based backup acceleration is something more and more vendors are pursuing. The la...

FalconStor and Network Engines' product is an appliance based on FalconStor's virtualization software that reportedly delivers three to four times improvements in backup and restore times, according to Bernie Wu, FalconStor VP of business development. Distributed through Network Engines subsidiary TidalWire, the virtual tape library appliance comes in either IP or Fibre Channel versions, supports up to 10 virtual tape libraries and 100 slots.

Unlike Quantum's DX line, the FalconStor/Network Engines VTL appliance is not tied to any particular disk subsystem. "We're just providing the virtual tape library head," says Wu. "The disk can come from virtually anywhere." As such, the appliance is "the least disruptive way to upgrade your existing infrastructure." It is priced at $15,900.

Overland Storage, meanwhile, is offering its Reo B2000, an iSCSI appliance based on technology the company gained when it bought Okapi Software this summer. Up to eight iSCSI servers can send backup to the B2000, with 100MB/s aggregate throughput, says John Matze, Overland vice president and CTO. The B2000 stores the data as a file, which can then gradually be sent off to a backup server, then tape. A 2TB B2000 is priced at approximately $25,000.

What do disk-based backup appliances bring to the table that generic IDE arrays don't? For one thing, explains Matze, with iSCSI you get the ability to share the appliance across multiple backup servers.

Perhaps more importantly, though, is the fact that the disk drives in the arrays behave like tape. Disk is inherently good at small block reads, explains Bryce Hein, ADIC business line manager for storage network appliances--not high-throughput writes. Therefore, a good portion of the intellectual property contained in enterprise-class disk backup products tunes the disk to behave like a streaming tape device.