Chapter 3. Storage virtualization 85
Figure 3-13 PPRC usage: cross cluster relationship
You can increase any one of these three licenses independently of the other.
That is you can increase the total amount of managed storage without increasing
the other licenses if the amounts of storage being copied remains unchanged.
Similarly, you can change the copy licenses independently of each other.
3.2.5 FlashCopy
This section looks at the fundamental principles of the operation of FlashCopy.
How it works
FlashCopy makes a copy of a set of source virtual disks to a set of target virtual
disks. The original contents of the target virtual disks are lost. After the copy
operation occurs, the target virtual disks have the contents of the source virtual
disks as they existed at a single point in time (PIT), known as a
T(0) copy. That is
to say that, although the copy operation takes finite time, the resulting data at the
target appears as though the copy was made instantaneously. This time is
several orders of magnitude less than the time which is required to copy the data
using conventional techniques.
As shown in Figure 3-14 on page 86, both the source and target virtual disks are
available for read and write operation, although not all the data is copied across
from the source to target volumes.
86 Understanding the IBM TotalStorage Open Software Family
Figure 3-14 Implementation of SVC FlashCopy
Rationale
Point-in-time copy techniques are used to help solve the problem where it is
difficult to make a consistent copy of a data set, which is being constantly
updated. If a copy of a data set is taken using a technology that did not provide
point-in-time semantics and the data set changes during the copy operation, then
the resulting copy may contain data that is not self consistent.
For example, a reference to an object may be copied earlier than the object itself
and the object is moved before it is itself copied. In this case, the copy contains
the referenced object at its new location, but the reference points to the old
location.
FlashCopy applications
The business applications for FlashCopy are many and various. An important
use is for taking consistent backups of changing data. In this application, a
FlashCopy is created to capture a point in time. The resulting image is backed up
to tertiary storage, such as tape. After the copied data is on tape, the FlashCopy
target is redundant.
Practical uses for FlashCopy include those that are explained in the following
sections.
Time
FlashCopy command is issued.
Copy is immediately available
after bitmaps (metadata) are built.
Read/write to copy is possible.
Write Read
Blocks that are not yet written to
the target are read from the source.
Before a write to the source, data is
copied to the target.
When a copy is complete, the
source and target are physically
available.
Source Target
Source Target

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