164 IBM System Storage Solutions for Smarter Systems
XIV system snapshot
The point-in-time-copy function in the XIV system is called snapshot. It is a block-level data
copy. XIV system snapshot is based on several innovative technologies that ensure minimal
degradation of storage system performance. XIV system snapshots make use of pointers and
do not necessarily copy all the data to the second instance of a volume. They efficiently share
cache for common data, effectively working as a larger cache than with full data copies. From
this perspective, the XIV system snapshot resembles the FlashCopy SE used in, for example,
DS8000 storage systems.
With FlashCopy SE, if there is a write I/O attempt to a block of data that has not been copied
yet since the creation of a relationship, a first copy of source volume data block is made, and
the target volume pointer now points to this new copy. Then the source volume is modified.
This is called
copy-on-write. XIV system uses a redirect-on-write mechanism. This means
that the pointer of the target volume is not altered, but new data is written to another location,
and the source volume pointer now points to this new block.
9.2.2 FlashCopy in combination with other Copy Services
For the DS8000, FlashCopy can work together with Metro Mirror and Global Mirror to provide
better data protection. (For an overview of the remote copy function, refer to “Remote Copy
Services for continuous operations” on page 188). For example, you can perform a Metro
Mirror copy to duplicate data from Site A to Site B, then perform a daily FlashCopy to copy the
FlashCopy with Metro Mirror or Global Copy
With this option, the FlashCopy target volume can also be a primary volume for a Metro Mirror
or Global Copy relationship. You can use this capability to create both a remote copy and a
local copy of a production volume.
Figure 9-3 illustrates this capability. The FlashCopy target and the Metro Mirror (or Global
Copy) primary are the same: volume B.
Figure 9-3 The FlashCopy target is the Metro Mirror (or Global Copy) primary
Site A Site B
Metro Mirror / Global Copy
Chapter 9. Backup and recovery 165
Remote Pair FlashCopy
Remote Pair FlashCopy is also called preserve mirror. It has been designed to overcome the
shortcomings of the previous solution to FlashCopy onto a Metro Mirror source volume and
the loss of the disaster recovery capability during the FlashCopy copy operation. When the
Metro Mirror copy is in a synchronous state (usually after initial synchronization or after a
switch from asynchronous Global Copy to synchronous Metro Mirror), the state of Metro
Mirror pairs is full duplex. During the FlashCopy, the Metro Mirror source volume, for a short
period of time, is not in full duplex, and thus disaster recovery is not ensured.
As the name implies, preserve mirror preserves the existing Metro Mirror status of full duplex
during the copy operation. Figure 9-4 shows this approach:
1. A FlashCopy command is issued by an application or by the customer to Local A volume
with Local B volume as the FlashCopy target.
2. The DS8000 firmware propagates the FlashCopy command through the Metro Mirror links
from the local storage server to the remote storage server. This in-band propagation of a
Copy Services command is only possible for FlashCopy commands.
Figure 9-4 Preserve Mirror operation scheme
Independent of each other, the local storage server and the remote storage server then
execute the FlashCopy operation. The local storage server coordinates the activities at its end
and takes action if the FlashCopy is not successful on both storage servers.
Global Mirror, as a long-distance remote copy solution, is based on an efficient combination of
Global Copy and FlashCopy functions. It is a transparent and autonomic mechanism to
intelligently utilize Global Copy in conjunction with certain FlashCopy operations to attain
consistent data at the remote site. Details of this copy solution are located in “IBM System
Storage DS8000 Global Mirror” on page 191.
Metro Mirror physical links
Site A Site B
Remote FlashCopy command
Remote BLocal B