Best practices for implementing disk-to-disk backup: Part 2

Computerworld - In this article, we continue our series on the best disk-to-disk backup strategies and wrap up our discussion on the challenges associated with software-based disk-to-disk backup.
As discussed in the previous article, software-based disk-to-disk backup creates some potential implementation and operation issues. We also noted that over time, backup software providers will resolve most of these issues.
The one area that backup software providers won't be able to address however, is the file system itself, specifically file system size, fragmentation and sharing. Unlike tape, in order for backup software to use disk as a backup destination, it must first have a file system installed on it. Ideally, this file system is large enough to hold the entire disk backup. If you have 10TB of backups, you'd like to make a 10TB file system.
The trouble with file systems
Many file systems, both practically and theoretically, cannot support anywhere close to this size. In fact, a 2TB file system size is large. Consequently, if we have 10TB of backup data and we can create only a 2TB file system, we will have to create five file systems. Each of these file systems must be independently managed and monitored, and more file systems must be created as the backup data set grows.
With disk-to-disk backup, fragmentation results from saving the backup jobs to the backup disk area. These jobs vary in size; they are made smaller, larger and eventually get deleted when you migrate the job to tape. This change and variation over time causes fragmentation. Since the backup-to-disk process is nothing but file changes and deletions, the resulting fragmentation happens faster and more severely than in other applications. All operating systems suffer from this problem, which can be solved only by using a disk defragmenter. However, using a disk defragmenter is very processor-intensive and unpopular with system administrators. With a multiple terabyte file system, a defragmentation job can run for days.
Another issue with a standard file system is its inability to be shared. In a tape storage-area network (SAN) environment, you can have multiple servers (even with different operating systems) accessing the same tape library at the same time. This is because each backup server using the same backup software writes its data stream to its own dedicated tape drive. This isn't true with a disk backup target on a standard file system, where multiple servers share the same disk destination at the same time. With a standard disk file system, each server performing backups needs its own file system on