But disk's role in backup may be changing. Last month, when EMC announced its Clariion Disk Library (DL), Chuck Hollis, EMC vice president of platform marketing, went to great pains to paint Clariion DL as distinct from virtual tape. Whereas virtual tape "is focused on helping your backup window, [Clariion DL] is about "improving your time to restore," he says.

However, not all backup software supports disk's role as a restore enhancer. According to Chris Van Wagoner, director of product marketing at backup software maker CommVault, most backup software is built on "the philosophical underpinning that disk is a short-term staging area for backup before it gets moved off to tape." In other words, for most backup software, a backup image can reside either on disk or on tape, but not on both.

If disk is to improve restore times, backup images must reside on the disk subsystem. If restore is your focus, but you still want to make off-site tapes, make sure that your backup software supports the notion of maintaining a backup image on both disk and tape, at the same time.

Some storage managers would like to be able to write their backups to disk and tape at the same time, instead of as a two-step process. InfoSure, a backup service provider in Vancouver, British Columbia, is planning to use the "asynchronous multi-write" feature offered by its backup software, Atempo's TimeNavigator. Currently, InfoSure relies on TimeNavigator to backup its customers' servers to a single Quantum DX30 virtual tape device. Going forward, InfoSure will send that same backup image to a secondary device--most likely tape--as part of a new archive service.

CommVault Galaxy will support what it dubs "in-line copy" in its next release, but will only be recommended for use with devices that are rated at the exact same speed. Legato NetWorker does not support streaming to multiple devices at all, says George Symons, Legato CTO, as it "constrains you to the speed of your tape device."