Chapter 7. Volumes 249
Each volume copy is not a separate object and can be manipulated only in the context of the 
volume. A mirrored volume behaves in the same way as any other volume; that is, all of its 
copies are expanded or shrunk when the volume is resized. The volume can participate in 
FlashCopy and remote copy relationships, is serviced by an I/O Group, and includes a 
preferred node. 
An overview of volume mirroring is shown in Figure 7-6.
Figure 7-6   Volume mirroring overview
A second copy can be added to a volume with a single copy or removed from a volume with 
two copies. Checks that are in place prevent accidental removal of the only remaining copy of 
a volume. A newly created, unformatted volume with two copies initially has the two copies in 
an out-of-synchronization state. The primary copy is defined as “fresh” and the secondary 
copy is defined as “stale”, and the volume is immediately available for use. 
The synchronization process updates the secondary copy until it is fully synchronized. This 
update is done at the synchronization rate that is defined when the volume is created (this 
volume parameter can be modified after volume creation). The synchronization status for 
mirrored volumes is recorded on the quorum disk.
If a mirrored volume is created by using the format parameter, both copies are formatted in 
parallel. The volume comes online when both operations are complete with the copies in 
sync.
If it is known that MDisk space (which is used for creating volume copies) is formatted or if the 
user does not require read stability, a no synchronization option can be selected that 
declares the copies as synchronized even when they are not.
If the mirrored volume needs resynchronization, the system copies to the out-of-sync volume 
copy only these 256 kibibyte (KiB) grains that were written to (changed) since the 
synchronization was lost. This approach is known as an incremental synchronization and it 
minimizes the time required to synchronize the volume copies.