IBM System storage DS6000 Series, Redbooks

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Chapter 6. Copy Services
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Figure 6-1 FlashCopy concepts
When a FlashCopy operation is invoked, the process of establishing the FlashCopy pair and
creating the necessary control bitmaps takes only a few seconds to complete. Thereafter, you
have access to a point-in-time copy of the source volume. As soon as the pair has been
established, you can read and write to both the source and the target volumes.
After creating the bitmaps, a background process begins to copy the real-data from the
source to the target volumes. If you access the source or the target volumes during the
background copy, FlashCopy manages these I/O requests as follows:
򐂰 Read from the source volume
When you read some data from the source volume, it is simply read from the source
volume.
򐂰 Read from the target volume
When you read some data from the target volume, FlashCopy checks the bitmaps and:
– If the backup data is already copied to the target volume, it is read from the target
volume.
– If the backup data is not copied yet, it is read from the source volume.
򐂰 Write to the source volume
When you write some data to the source volume, at first the updated data is written to the
data cache and persistent memory (write cache). And when the updated data is destaged
to the source volume, FlashCopy checks the bitmaps and:
– If the backup data is already copied, it is simply updated on the source volume.
– If the backup data is not copied yet, first the backup data is copied to the target volume,
and then it is updated on the source volume.
򐂰 Write to the target volume
When you write some data to the target volume, it is written to the data cache and
persistent memory, and FlashCopy manages the bitmaps to not overwrite the latest data.
FlashCopy does not overwrite the latest data by the physical copy.
FlashCopy provides a point-in-time copy
FlashCopy command issued
Ti
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e
Copy immediately available
Read and write to both source
and copy possible
Write Read
When copy is complete,
relationship between
source and target ends
Source Target
Read
Write
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