IBM TotalStorage DS300 Best Practices Manual page 190

Figure 4-152 Array with Flashcopy source and target logical drive
The Flashcopy target logical drive can now be assigned to an other host than the
source logical drive is assigned to. This is important because the date which can
be read from both logical drives is identical. This means that the disk signature
written by a Microsoft Windows operating system is also identical. This will cause
problems when the Flashcopy target logical drive is assigned to the same host.
To read data from the Flashcopy target the controller is looking into the
Copy-On-Write lookup table. This table explains if the requested block of data is
already in the COW data area or must be read from the source logical drive. After
the creation of the Flashcopy target logical drive, no data block are in the COW
area and all blocks are read from the source logical drive.
Before a block with new data on the source logical drive is overwritten a
background task in the storage enclosure will copy this block with the old data to
the Flashcopy target logical drive. The lookup table is modified to reflect that the
data block is now on the Flashcopy target logical drive. The relationship to the
data block on the source logical drive is not longer required and removed. Future
changes of this data block on the source logical drive will not be copied to the
target logical drive. This kind of Flashcopy is also called Copy-On-Write (COW)
because the data copy occurs as soon as a write to a data block is performed.
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IBM TotalStorage DS300 and DS400 Best Practices Guide