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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.
You can save even 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 service contracts should be resumed if you're using the unit in a production system. Since service contracts on older equipment can be quite 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. This article addresses how to use disk to augment a traditional backup system. A subsequent article will concentrate on new types of backup systems, such as data-reduction backup and replication-based backup. A data-reduction backup system tries to eliminate redundant blocks of backed up data, thus 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.
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.
A disadvantage of disk-as-disk backup devices is the nature of filesystems. Files are written, opened, changed and stored back to the same place. Often, the new version of the file doesn't fit in the same place where the old file was, so a portion of it gets written to the original location while another part is written somewhere else on the disk, resulting in fragmentation. The more files you add, delete and modify, the more fragmented the filesystem. The way a backup system uses the disk results in significant fragmentation over time, which degrades performance.
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 allow you to scan that tape, figure out 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 copy their disk-based backups to tape. 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 you to purchase 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. (It should be noted that in-band, software-based compression products typically come with a rather hefty performance penalty--as much as 50%. In its new DX100 VTL, Quantum Corp. claims to offer hardware-based compression that doesn't degrade performance.)
This was first published in February 2005