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Disadvantages of disk-as-disk targets
Backup software companies are starting to charge for backing up to a disk-as-disk target, a trend that's expected to continue. Vendors defend this move because they're providing additional functions to their backup software. The going price to use a disk array as a staging device before data is moved to tape is approximately $2,000/TB. To use a 200TB disk array as a disk-as-disk target could add $400,000 to your backup software tab.
|D2D2T backup terms|
Another issue when using disk-as-disk backup targets is that some backup software products don't back up to filesystems as well as they back up to tapes. For example, backup software products know exactly what to do when a tape fills up, but they're not always sure what to do when a filesystem fills up. Many of the major backup products require users to point disk-as-disk backups to a single filesystem. When that filesystem fills up, all the backups fail--even if another filesystem has adequate capacity. There are also other limitations, like the inability of some backup products to scan in filesystem images. If you let a tape expire from your backup catalog, most backup products will let you scan the tape, find what's on it and then enter its contents in the backup catalog. Some products can't do that with filesystem-based images.
Storing backups offsite is another challenge with disk-as-disk backup targets. The normal procedure would be to copy the disk-based backups to a physical tape and then ship the tape offsite. The problem is that most people don't currently create copies of their tape-based backups; they'd have to start doing so if they started backing up to a disk-as-disk target. Therefore, you need to learn how to copy disk-based backup data to tape and then learn how to automate the process. These two steps can range from extremely easy to extremely difficult, depending on the backup product you use, and may also require the purchase of additional software from your backup vendor. Whatever method you choose to get the data from disk to tape, remember that the data is now moving twice, where before it moved only once. This means you'll need to budget time for the data to make that second move.
One final disadvantage of disk-as-disk targets is the lack of compression. While there's currently one NAS disk-as-disk target that uses data-reduction techniques on backups stored on that device (Data Domain Inc.'s DD200), most disk-as-disk targets don't have built-in compression. This means you may need twice as much disk with a disk-as-disk target as you would with a VTL that supports compression. (In-band, software-based compression products typically come with a rather hefty performance penalty--as much as 50%. In its DX100 VTL, Quantum Corp. claims to offer hardware-based compression that doesn't degrade performance.)
This was first published in November 2005