VTLs emerged when disk backup was first introduced and backup software was designed to write data to a tape device. By emulating tape drives and tape libraries, VTLs were originally 'plug replacements' for these storage devices. In reality, they weren't always so easy to use.
Device emulation in electronics is a common connectivity solution, but it can bring its own problems, especially in larger environments. SCSI timeouts and the inability to read and write to a virtual tape drive simultaneously are two issues, plus managing a virtual drive requires much more effort than managing a set of backup files on a NAS.
So, when is a VTL a better option for a backup target?
Larger, Fibre Channel environments. VTLs typically offer FC for connectivity, which is more efficient and offers better performance (and lower server overhead) than Ethernet, something that can be an issue for large data transfers. Many of these environments are already 'plumbed' with FC, so an FC-based backup solution like a VTL is a natural.
Disk-to-disk-to-tape (D2D2T). VTLs support D2D2T better than NAS when disk is the initial backup target, especially when multiple servers are sharing that disk. VTLs can write directly to tape; NAS solutions may require that data is resent through the backup server.
Deduplication. When data must be kept on disk for extended periods of time or very large data sets are being backed up to disk, deduplication can reduce the cost of the solution significantly.
VTLs offer integrated deduplication, which doesn't require dedupe to be run in the backup software. This can lessen the potential impact on server performance and may provide better dedupe functionality than software-based dedupe that comes with backup applications. Dedupe can also be run on a NAS device but unless the product is designed as a backup appliance, the dedupe technology may not be as flexible or efficient as VTL dedupe.
In general, a VTL is a better fit for larger environments where the amount of data to be backed up is significant and the time required to do so may be restricted. NAS devices, especially those that have deduplication built in, are ideal for midsize or smaller environments and those where tape is being de-emphasized.
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