Understanding the IBM TotalStorage Open Software Family

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