IBM TotalStorage DS300 Best Practices Manual page 191

Figure 4-153 Array with Flashcopy source and target logical drive - Changed data on
source logical drive
When the Flashcopy target logical drive was in a read/write mode assigned to an
host system the data on the target logical drive can be changed. Reading of data
is the same under a read/write conditions. When data is not in the COW area it
will be taken from the source logical drive. Writing the data in the COW area will
just change this data. When the data was read from the source logical drive and
is written to the Flashcopy target logical drive the COW lookup table will be
modified and the relationship between the data blocks dissolved.
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Chapter 4. TotalStorage DS300 and DS400 management