Global-active device (GAD) enables you to create and maintain synchronous, remote copies of data volumes.
A virtual storage machine is configured in the primary and secondary storage systems using the information of the primary storage system, and the global-active device (GAD) primary and secondary volumes are assigned the same virtual LDEV number in the virtual storage machine. This enables the host to see the pair volumes as a single volume on a single storage system, and both volumes receive the same data from the host.
A quorum disk, which can be located in a third and external storage system, is used to monitor the GAD pair volumes. The quorum disk acts as a heartbeat for the GAD pair, with both storage systems accessing the quorum disk to check on each other. A communication failure between systems results in a series of checks with the quorum disk to identify the problem for the system able to receive host updates.
Alternate path software on the host runs in the Active/Active configuration. While this configuration works well at campus distances, at metro distances you must use multi-pathing software with ALUA (asymmetric logical unit access) or Hitachi Dynamic Link Manager to support preferred/nonpreferred paths and ensure that the shortest path is used.
If the host cannot access the primary volume (P-VOL) or secondary volume (S-VOL), host I/O is redirected by the alternate path software to the appropriate volume without any impact to the host applications.
Global-active device provides the following benefits:
- Continuous server I/O when a failure prevents access to a data volume
- Server failover and failback without storage impact
- Migrating virtual machines without storage impact
