FUJITSU CentricStor V3.1D - Manual (Page 24)

24 U41117-J-Z125-7-76
The CentricStor principle CentricStor - Virtual Tape Library
Host robot system with CentricStor
Figure 2: Host robot system with CentricStor
With CentricStor, a virtual archive system is installed upstream of the real archive system
with the physical drives and data cartridges. This enables the host to be completely isolated
from the real archive. The virtual archive system contains a series of logical drives and
volumes. At its heart is a data buffer, known as the disk cache, in which the logical volumes
are made available. This guarantees extremely fast access to the data, in most cases
allowing both read and write operations to be performed much more efficiently than in
conventional operation.
Instead of the term logical drives (or volumes), the term virtual drives (or volumes)
is sometimes also used. These terms should be regarded as synonyms. In this
manual the term logical is used consistently when drives and volumes in
CentricStor are meant, and physical when the real peripherals are meant.
The virtual archive system is particularly attractive, as it provides a large number of logical
drives compared to the number of physical drives. As a result, bottlenecks which exist in a
real archive can be eliminated or avoided.
From the host’s viewpoint, the logical drives and volumes act like real storage peripherals.
When a mount job is issued by a mainframe application or an open systems server, for
example, the requested logical volume is loaded into the disk cache. If the application then
writes data to the logical drive, the incoming data stream is written to the logical volume
created in the disk cache.
The Library Manager of the virtual archive system then issues a mount job to the real
archive system asynchronously and completely transparently to the host. The data is read
out directly from the disk cache and written to a physical tape cartridge. The physical
volume is thus updated with optimum resource utilization.
Logical volumes in the disk cache are not erased immediately. Instead, data is displaced in
accordance with the LRU principle (Least Recently Used). Sufficient space for this must be
allocated in the disk cache.
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