Virtual Tape Technology

Disk drive technology in the form of ATA has now reduced in cost to the extent that it is feasible to use it as an alternative to tape. At the time of writing, individual disk capacities of 1TB are commonplace and larger sizes will shortly be introduced in the marketplace. Many tape vendors are implementing disk based storage systems which appear to the backup application software as tape library devices. The terminology Virtual Tape is applied to these devices. There are essentially three parts to a Virtual tape system –

· The storage medium

· The emulation software

· The Backup application

Disk array vendors are providing high capacity enclosures with 16TB or greater in a small form factor. These arrays house the disks which provide the media to store the data.

The emulation software is the component that makes the drive appear as a tape device to the application software. This is an extremely important component and disk emulation software must have the ability to appear as a number of different tape standards such as LTO etc. It must deal with commands such as rewind in a logical fashion and provide virtual tape cartridges and bar coding for robotic libraries. Other desirable capabilities include the ability to provide data compression within hardware as this will not only reduce the storage size required but will potentially speed up data transfers as less data has to be stored.

The virtual tape then should work seamlessly with the disk backup application. The fact that a disk array exists must be totally transparent to the application. Operating System distributions such as Linux often include Snapshot applications which would allow turnkey solutions to be delivered to users.

Rather than emulate tape libraries, many organizations may find that disk based backup provides an adequate solution. The speed and simplicity benefits may be sufficient and simple data synchronization software can ensure that mirrored copies are kept on a secondary volume. This data can then in turn be saved to a conventional tape system since tapes provide the benefit of removable media and off site security. Disk to Disk backup systems are sometimes referred to as D2D and Disk to Disk to Tape may be referred to as D2D2T.

What else should disk array vendors consider when entering this market apart from emulation software and software providers/agents to do the work? One area is to perform the backup operation independent of any server. Ideally peripherals should be able to transfer data from one device to another independent of the server, in the same fashion that intelligent controllers perform DMA transfers without processor intervention. A command exists within the SCSI protocol known as Extended Copy to do this. An array which implements this command within firmware would be desirable as this would provide more efficient transfers. This is termed Serverless Backup and it provides autonomy for backup operations.