IBM System storage DS6000 Series, Redbooks

Page: 119 / 406

Chapter 6. Copy Services
95
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 can 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 issues 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 6-5.
Figure 6-5 Consistency Group FlashCopy
Server1
Server2
write requests
LSS11
LSS12
Wait
Wait
FlashCopy
FlashCopy
LSS11
LSS12
Wait
Wait
FlashCopy
FlashCopy
LSS11
LSS12
Wait
Wait
FlashCopy
FlashCopy
Consistency Group
Waiting write operation
until Consistency Group Created
command is invoked.
DS6000 #1, #2, #3, …