IBM DS8000 - Manual (Page 139)

Chapter 7. Copy Services 117
See Figure 7-1 for an illustration of FlashCopy concepts.
Figure 7-1 FlashCopy concepts
When a FlashCopy operation is invoked, it takes only a few seconds to complete the process
of establishing the FlashCopy pair and creating the necessary control bitmaps. 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 target volumes.
After creating the bitmap, 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 bitmap 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 bitmap and:
– If the backup data is already copied, it is simply updated on the source volume.
Note: In this chapter, track means a piece of data in the DS8000; the DS8000 uses the
logical tracks to manage the Copy Services functions.
FlashCopy provides a point-in-time copy
FlashCopy command issued
Ti
m
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
T
0