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There are four ways to add disk to a traditional backup system. The first two options are called disk-as-disk because they involve using disk drives behaving as disk drives--the disks aren't pretending to be tape. In a SAN disk-as-disk configuration (see "SAN disk-as-disk," this page), a disk array is connected to one or more backup servers via a SAN, and a disk volume is assigned to each server. Each server then puts a filesystem on that volume, and backups are sent to that filesystem. In a NAS disk-as-disk architecture (see "NAS disk-as-disk," this page), the disk resides behind a filer head that shares filesystems via NFS or CIFS, and backups are sent to those filesystems.
The last two options employ virtual tape libraries (VTLs), where disk systems are placed behind a server running software that lets the disk array pretend to be one or more tape libraries. "Standalone virtual tape library" (this page) shows standalone VTLs that sit next to a physical tape library and pretend to be another tape library. Once you back up to a standalone VTL, you must use the backup server to copy its backups to physical tape if you want to send them offsite. An integrated VTL (see "Integrated virtual tape library," this page) sits between a physical tape library and a backup server, where it pretends to be a physical library. The backup server backs up to the integrated VTL, which then copies the data to the physical tape portion of its library.
When backup software backs up to a disk-as-disk system, it knows it's a disk and typically creates a file within the filesystem. To distinguish these backups from those sent to a tape (or virtual tape) target, some people refer to these types of backups as filesystem-based backups.
Advantages of disk-as-disk targets
The biggest advantage disk-as-disk targets have over most VTL targets is price. Most disk-as-disk systems are priced significantly less per gigabyte than VTL systems because you're paying for the value of the VTL software.
It's possible to save more money by redeploying an older, decommissioned array as a disk-as-disk target. Decommissioned arrays are often end-of-life units without service contracts, so these contracts should be resumed if you're using the unit in a production system. (Service contracts on older equipment can be expensive; be sure to compare the cost of resuming the contract to that of a new system with a contract included.) Another advantage of disk-as-disk backup targets is that most backup software companies don't currently charge to back up to them; unfortunately, this is changing.
The final advantage of disk-as-disk targets is their flexibility, which may come into play if you plan on moving away from a traditional backup architecture to, for example, a data-reduction backup or a replication-based backup system. A data-reduction backup system tries to eliminate redundant blocks of backed up data, reducing the amount of data sent across the network and stored on the secondary storage system. A replication-based backup system uses replication as the mechanism to move data to a secondary location where it's then backed up. If one of these two new architectures is possibly in your future, you might want to consider a disk-as-disk target now; one advantage of disk-as-disk targets is that they're exactly what data-reduction backup systems and replication-based backup systems need as a target. You can't replicate to tape, and data reduction backup systems are also designed to go to disk-as-disk.
This was first published in November 2005