IBM DS8000 manual What is Consistency Group FlashCopy?

Consistency Group FlashCopy

Consistency Group FlashCopy allows you to freeze (temporarily queue) I/O activity to a LUN or volume. Consistency Group FlashCopy helps you to create a consistent point-in-time copy across multiple LUNs or volumes, and even across multiple storage units.

What is Consistency Group FlashCopy?

If a consistent point-in-time copy across many logical volumes is required, and the user does not wish to quiesce host I/O or database operations, then the user may use Consistency Group FlashCopy to create a consistent copy across multiple logical volumes in multiple storage units.

In order to create this consistent copy, the user would issue a set of Establish FlashCopy commands with a freeze option, which will hold off host I/O to the source volumes. In other words, Consistency Group FlashCopy provides the capability to temporarily queue (at the host I/O level, not the application level) subsequent write operations to the source volumes that are part of the Consistency Group. During the temporary queueing, Establish FlashCopy is completed. The temporary queueing continues until this condition is reset by the Consistency Group Created command or the time-out value expires (the default is two minutes).

Once all of the Establish FlashCopy requests have completed, a set of Consistency Group Created commands must be issued via the same set of DS network interface servers. The Consistency Group Created commands are directed to each logical subsystem (LSS) involved in the consistency group. The Consistency Group Created command allows the write operations to resume to the source volumes.

This operation is illustrated in Figure 7-5.

		DS8000 #1, #2, #3, …
Server1		LSS11
			LSS11
				LSS11
Server2		Wait	FlashCopy
	write requests	Wait		FlashCopy
		Wait		FlashCopy
		LSS12
			LSS12
				LSS12
		Wait	FlashCopy
		Wait		FlashCopy
Waiting write operation		Wait		FlashCopy
until Consistency Group Created		Consistency Group
command is invoked.

Figure 7-5 Consistency Group FlashCopy

Chapter 7. Copy Services 121