Stretched systems

A stretched system is an extended high availability (HA) method that is supported by SAN Volume Controller systems to enable I/O operations to continue after the loss of half of the system.

A stretched system is also sometimes referred to as a split system because one half of the system and I/O group is usually in a geographically distant location from the other, often 10 kilometers or more. A third site is required to host a storage system that provides a quorum disk.

Enhanced stretched systems

The system supports an enhanced stretched system configuration. This configuration can be used regardless of whether the stretched system is configured with or without ISLs between nodes.

Enhanced stretched systems provide the following primary benefits:

In addition to the automatic failover that occurs when a site fails in a standard stretched system configuration, an enhanced stretched system provides a manual override that can be used to choose which of two sites continues operation.
Enhanced stretched systems intelligently route I/O traffic between nodes and controllers to reduce the amount of I/O traffic between sites, and to minimize the impact to host application I/O latency.
Enhanced stretched systems include an implementation of extra policing rules to ensure the correct configuration of a standard stretched system.

Note: In an enhanced stretched system, set the site attribute in the node and controller object.