IBM DS8000 Read from the source volume, Read from the target volume, Write to 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.

See Figure 7-1for an illustration of FlashCopy concepts.

FlashCopy provides a point-in-time copy

Time

Source Target

Write Read	Read
	Write

T0

FlashCopy command issued

Copy immediately available

Read and write to both source and copy possible

When copy is complete, relationship between source and target ends

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.

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