The result is that many affordable backup options are available for small and midsize businesses as well as enterprise-level departments. D2D2T (disk-to-disk-to-tape) appliances let backups run to disk and then, over time, stream data off to tape, enabling your core systems to back up using fast disk-to-disk techniques, and off-loading the slower disk-to-tape backups to the appliances. Depending on the technology and vendor you use, disk-to-disk backups can be two to five times faster than disk-to-tape backups. More important, restores in those first few hours between backup and when the data is streamed to tape occur at random-seek disk speeds instead of streaming tape speeds.
The benefits of these systems over conventional low-end backup methodologies are compelling. First, the speed advantages mean you lose less network bandwidth and processing time on your servers. Second, most restore requests are resolved quickly. You need only last night's backups to fix this type of problem, and last night's backups should still be sitting on disk--faster restores mean getting the business back up and running quickly. Finally, you can (and should) store tapes off-site, meaning you have a backup even if your entire company suffers a catastrophe (see "Disk, Tape and the Better Backup,").
Most D2D2T appliances work by presenting themselves as tape drives. They accept the commands of major backup software systems just as if they were a tape archive, then stream the associated data to disk. On a regular schedule that you set, they then move the backups off to tape and delete the disk-based copy on a separate schedule, though some delete at the end of the tape backup process. Of course, you still must determine what gets backed up and when, and configure your servers to perform the backups. In addition, you must take the time to move the tapes off-site, as well as check your backups regularly to ensure they're valid.
More and more D2D2T devices are offering iSCSI targets. Whether you prefer iSCSI or CIFS and NFS is up to you, but bear in mind that D2D2T appliances require backups to be over the network. If your backups have been local to your machines, make certain your network can handle the additional traffic before you spend money. For more on iSCSI, see our iSCSI SAN Buyer's Guide, as well as "State of the Art: How To Build an iSCSI SAN".