The Metro Mirror and Global Mirror Copy Services features enable you to set up a relationship between two virtual disks (VDisks), so that updates that are made by an application to one VDisk are mirrored on the other VDisk. The VDisks can be in the same cluster or on two different clusters.
Although the application only writes to a single VDisk, the SAN Volume Controller maintains two copies of the data. If the copies are separated by a significant distance, the Metro Mirror and Global Mirror copies can be used as a backup for disaster recovery. A prerequisite for the SAN Volume Controller Metro Mirror and Global Mirror operations between clusters is that the SAN fabric to which they are attached provides adequate bandwidth between the clusters.
For both Metro Mirror and Global Mirror copy types, one VDisk is designated the primary and the other VDisk is designated the secondary. Host applications write data to the primary VDisk, and updates to the primary VDisk are copied to the secondary VDisk. Normally, host applications do not perform I/O operations to the secondary VDisk.
The Metro Mirror feature provides a synchronous-copy process. When a host writes to the primary VDisk, it does not receive confirmation of I/O completion until the write operation has completed for the copy on both the primary VDisk and the secondary VDisk. This ensures that the secondary VDisk is always up-to-date with the primary VDisk in the event that a failover operation must be performed. However, the host is limited to the latency and bandwidth limitations of the communication link to the secondary VDisk.
The Global Mirror feature provides an asynchronous-copy process. When a host writes to the primary VDisk, confirmation of I/O completion is received before the write operation has completed for the copy on the secondary VDisk. If a failover operation is performed, the application must recover and apply any updates that were not committed to the secondary VDisk. If I/O operations on the primary VDisk are paused for a small length of time, the secondary VDisk can become an exact match of the primary VDisk.