A virtual tape system uses tape libraries with a disk cache controlled by a server running storage management software. Like virtual disk storage, virtual tape storage breaks the direct connection between logical drives and physical drives, allowing more efficient use of resources and a cheaper overall solution. Among other advantages, an IDC survey of virtual tape systems showed that the average user reduced the number of tape cartridges needed by more than 25 percent.
The "gotcha" is that a virtual tape storage system needs to be carefully sized to avoid major performance degradation. Because all the virtualized data is flowing through one system, any bottleneck has the potential to affect not just one drive or library, but every tape on the system. The most common problems are not having enough tape drives or not having enough disk cache.
IDC's advice is to do it right or not use virtual tape systems at all. "Users may be better advised to forestall an investment that is too small until the funds can be found to acquire the properly sized solution," IDC says. "The ill will between supplier and the IT department and the IT department and its users that comes from an inadequately size solution is not worth the initial capital savings."
An IDC white paper titled "Virtual Tape Servers, a Foundation for Growth" is available at IBM's storage web site at:
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
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This was first published in July 2002