Backing Up Oracle - Computing at Cornell

Figure 59. FlashCopy concept
There are two requirements to allow a FlashCopy relation between two ESS
volumes to be established:
The volumes must be in the same LSS (logical subsystem). LSS is an
internal ESS structure that groups certain ESS volumes together. For a
description on LSS, see Appendix B, “Understanding logical subystems” in
the redbook Implementing ESS Copy Services on UNIX and Windows
NT/2000, SG24-5757-00.
The target volume must be equal to or greater in size than the source
volumes. We recommend that you select target volumes with exactly the
same size as the source volumes.
At a specific time, the FlashCopy relationship between source and target
volumes will be established. This is known as the T 0 (time zero) copy. The
process of establishing this T 0 copy takes only a few seconds. A bitmap will
be created to identify the T 0 data on the source disk.
At the time that the FlashCopy is started, the target volume is basically
empty. The background copy task copies data from the source to the target.
The FlashCopy bitmap keeps track of which data has been copied from
source to target. If an application wants to read some data from the target
that has not yet been copied to the target, the data is read from the source;
otherwise, the read is satisfied from the target volume. See Figure 60.
228 Backing Up Oracle using Tivoli Storage Management