Advantages of VTL tape virtualization
Operating systems and backup applications that currently interact with a physical library via ACSLS continue to use ACSLS when accessing VTL virtual libraries. ACSLS further hides the particulars of disk‐based tape virtualization behind a standard, well‐known interface.
This ability to mask the library implementation from client applications and operating systems lets ACSLS make disk‐based tape‐virtualization available to platforms that could not otherwise support it. Platforms that do not support SCSI passthrough library control, such as Unisys, Tangent, and mainframe operating systems, can back up to Sun VTL virtual libraries using ACSLS.
The more powerful, more specialized library‐control functionality of ACSLS improves the availability of suitably configured VTL solutions. The brief SCSI path interruptions that may accompany failover and failback are less likely to interfere with the backup application, because ACSLS retries the connection aggressively enough to restore the path.
True tape virtualization with dynamically allocated disk space
Correctly configured, dynamically sized virtual tape volumes provide the highest capacity and performance. When tapes are created with the VTL Capacity On Demand feature enabled, the VTL software allocates space as data is written to disk rather than all at once. For instance, a physical tape with a capacity of 400‐GB can be emulated without allocating any space initially, and thereafter enlarged as needed in 5‐GB increments (see the figure below).
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| = | = |
Space is not allocated on disk until data is written
-GB
virtual
02B10001 
400-GB
This approach to space allocation has two major advantages. First, it minimizes wasted disk capacity. Second, and perhaps more importantly, it maximizes array performance and reliability. Dividing data into multiple segments distributes it more evenly across the array, involves more volume groups in each I/O, and reduces the average length of each seek during I/O.
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