A SAN Volume Controller cluster has several features that can be used to deploy a high availability storage system with no single point of failure.
Each I/O group within a cluster consists of a pair of nodes. If a node fails within an I/O group, the other node in the I/O group assumes the I/O responsibilities of the failed node. If the node contains solid-state drives (SSDs), you should create a mirrored virtual disk (VDisk) of any VDisk that uses the SSDs. SSDs can be a single point of failure in the event of an outage to the SSDs or to the node itself.
If a cluster of SAN Volume Controller nodes is split into two partitions (for example due to a SAN fabric fault), the partition with the majority of nodes continues to process I/O operations. If a cluster is split into two equal-sized partitions, a quorum disk is accessed to determine which half of the cluster continues to read and write data.
Each SAN Volume Controller node has four fibre-channel ports, which can be used to attach the node to multiple SAN fabrics. For high availability, attach the nodes in a cluster to at least two fabrics. SAN Volume Controller software incorporates multipathing software that is used for communication among SAN Volume Controller nodes and for I/O operations among SAN Volume Controller nodes and storage systems. If a SAN fabric fault disrupts communication or I/O operations, the multipathing software recovers and retries the operation through an alternative communication path. Also for high availability, configure your fibre-channel host systems to use multipathing software. If a SAN fabric fault or node failure occurs, I/O operations among fibre-channel host systems and SAN Volume Controller nodes are retried. Subsystem device driver (SDD) multipathing software is available from IBM at no additional charge for use with SAN Volume Controller. For additional information about subsystem device driver (SDD), go to the Support for IBM Systems Web site:
iSCSI-attached hosts connect to SAN Volume Controller through node Ethernet ports. If a node fails, SAN Volume Controller maintains host availability by failing over the IP addresses of the failed node to the partner node in the I/O group.
The SAN Volume Controller Virtual Disk Mirroring feature can be used to mirror data across storage systems. This feature provides protection against a storage system failure.
The SAN Volume Controller Metro Mirror and Global Mirror features can be used to mirror data between clusters at different physical locations for disaster recovery.