Sun Microsystems Virtual Tape Library - Manual (Page 21)

Advantages of VTL tape virtualization
96267 • G Chapter 1 Introduction: VTL appliances and enterprise data-protection 7
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).
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.
Space is not allocated on
disk until data is written
400-GB physical tape 01A00001
400-GB physical tape 01A00002
400-GB virtual tape 02B10001
= 5-GB header/data segment = 5-GB data segment = 2-MB tape archive files
400-GB virtual tape 02B10000