Chapter 11. Advanced Copy Services 523
IBM Spectrum Virtualize supports the resynchronization of changed data so that write failures 
that occur on the master or auxiliary volumes do not require a complete resynchronization of 
the relationship. 
11.6.8  Metro Mirror attributes
The Metro Mirror function in IBM Spectrum Virtualize possesses the following attributes:
/SM590000A partnership is created between two IBM Spectrum Virtualize systems operating in the 
replication layer (for intercluster Metro Mirror).
/SM590000A Metro Mirror relationship is created between two volumes of the same size.
/SM590000To manage multiple Metro Mirror relationships as one entity, relationships can be made 
part of a Metro Mirror Consistency Group, which ensures data consistency across multiple 
Metro Mirror relationships and provides ease of management.
/SM590000When a Metro Mirror relationship is started and when the background copy completes, the 
relationship becomes consistent and synchronized.
/SM590000After the relationship is synchronized, the auxiliary volume holds a copy of the production 
data at the primary, which can be used for DR.
/SM590000The auxiliary volume is in read-only mode when relationship is active.
/SM590000To access the auxiliary volume, the Metro Mirror relationship must be stopped with the 
access option enabled before write I/O is allowed to the auxiliary.
/SM590000The remote host server is mapped to the auxiliary volume, and the disk is available for I/O.
11.6.9  Practical use of Metro Mirror
The master volume is the production volume, and updates to this copy are mirrored in real 
time to the auxiliary volume. The contents of the auxiliary volume that existed when the 
relationship was created are deleted.
While the Metro Mirror relationship is active, the auxiliary volume is not accessible for host 
application write I/O at any time. The IBM Storwize V7000 allows read-only access to the 
auxiliary volume when it contains a consistent image. IBM Storwize allows boot time 
operating system discovery to complete without an error, so that any hosts at the secondary 
site can be ready to start the applications with minimum delay, if required.
For example, many operating systems must read LBA zero to configure a logical unit. 
Although read access is allowed at the auxiliary in practice, the data on the auxiliary volumes 
cannot be read by a host because most operating systems write a “dirty bit” to the file system 
when it is mounted. Because this write operation is not allowed on the auxiliary volume, the 
volume cannot be mounted.
This access is provided only where consistency can be ensured. However, coherency cannot 
be maintained between reads that are performed at the auxiliary and later write I/Os that are 
performed at the master.
Switching copy direction: The copy direction for a Metro Mirror relationship can be 
switched so that the auxiliary volume becomes the master, and the master volume 
becomes the auxiliary, which is similar to the FlashCopy restore option. However, although 
the FlashCopy target volume can operate in read/write mode, the target volume of the 
started remote copy is always in read-only mode.