Weigh the need for disk storage against current processes. Before incurring the expense of a new VTL platform, consider the current backup window, recovery point objective (RPO) and recovery time objective (RTO). If backup objectives are met with tape, the addition of a VTL may be difficult to justify -- the added performance of disk storage would have no business impact. Organisations that are already leveraging other disk storage technologies with snapshot or replication tools may also see little benefit from VTLs. However, VTLs may be a worthwhile technology to shorten backup windows, RPOs and RTOs, where tape is currently in use. For example, an organisation experiencing backup windows that routinely run over into production time (e.g. running through a weekend into Monday morning) may see a significant benefit from a VTL.
Consider the compatibility and interoperability of the VTL platform. VTL systems and backup software are not universally compatible. Be sure that any prospective VTL system will be compatible with your existing backup software, such as LegatoNetWorker or IBM Tivoli. In many cases, the VTL will emulate a number of tape drives (e.g., LTO-2 or LTO-3), which the backup software can access directly, requiring only a relatively minor reconfiguration of the backup software.
Consider VTL storage capacity versus backup needs. VTL storage capacity is not endless -- VTLs only implement a finite amount of disk space. When planning a VTL, understand the space needed for each backup and then multiply that by the length of time those backups need to be retained. For example, if you're backing up 5 terabytes (TB) each week and need to retain the backups for at least four weeks, the VTL would need at least 20 TB of capacity. Many users opt for considerably more capacity to meet burgeoning storage needs. Long-term storage scalability is often an overlooked attribute of VTL platforms.
Look for data compression technologies to extend disk capacity. As with ordinary disk arrays, compression techniques are emerging to fit more data on less disk space. In addition to conventional compression, which removes redundant data from each file, data deduplication provides "single instance" storage so that only one copy of a file or block is actually stored on the VTL. For a VTL, this increases the number of backups that can be stored locally.
Consider long-term archival storage of backed up data. A VTL cannot hold a backup set forever. Once a backup set "ages out," it must be offloaded from the VTL to make space for new backups. In many cases, it's not acceptable to simply delete the backup set. Instead, the backup is offloaded to archival storage on a SATA disk array or tape library. The tape system that was displaced by a VTL is often redeployed as an archival storage system to create a disk-to-disk-to-tape (D2D2T) architecture.
The VTL specifications page in this chapter covers the following products: