For high availability, you can split a SAN Volume Controller cluster across three locations and mirror the data.
To provide protection against failures that affect an entire location, such as a power failure, you can use a configuration that splits a single SAN Volume Controller cluster across three physical locations. However, you must consider that split clusters typically exhibit substantially reduced performance.
- Directly connect each SAN Volume Controller node to one or more SAN fabrics at the primary and secondary sites. Sites are defined as independent power domains that would fail independently. Power domains could be located in the same room or across separate physical locations.
- Use a third site to house a quorum disk.
- The storage system that provides the quorum disk at the third site must support extended quorum disks. Storage systems that provide extended quorum support are listed at the following Web site:
- Do not use powered devices to provide distance extension for the SAN Volume Controller to switch connections.
- Place independent storage systems at the primary and secondary sites, and use VDisk mirroring to mirror the host data between storage systems at the two sites.
- SAN Volume Controller nodes that are in the same I/O group and separated by more than 100 meters (109 yards) must use longwave fibre-channel connections. Longwave SFPs can be purchased as an optional SAN Volume Controller component, and must be one of the longwave SFPs listed at the following Web site:
- Using inter-switch links (ISLs) in paths between SAN Volume Controller nodes in the same I/O group is not supported.
- Avoid using inter-switch links (ISLs) in paths between SAN Volume Controller nodes and external storage systems. If this is unavoidable, do not oversubscribe the ISLs because of substantial fibre-channel traffic across the ISLs. For most configurations, trunking is required. Because ISL problems are difficult to diagnose, switch-port error statistics must be collected and regularly monitored to detect failures.
- Using a single switch at the third site can lead to the creation of a single fabric rather than two independent and redundant fabrics. A single fabric is an unsupported configuration.
- SAN Volume Controller nodes in the same cluster must be connected to the same Ethernet subnet.
- A SAN Volume Controller node must be located in the same rack as the 2145 UPS or 2145 UPS-1U that supplies its power.
- Some service actions require physical access to all SAN Volume Controller nodes in a cluster. If nodes in a split cluster and separated by more than 100 meters, service actions might require multiple service personnel. Contact your IBM service representative to inquire about multiple site support.
A split cluster configuration locates the active quorum disk at a third site. If communication is lost between the primary and secondary sites, the site with access to the active quorum disk continues to process transactions. If communication is lost to the active quorum disk, an alternative quorum disk at another site can become the active quorum disk.
Although a cluster of SAN Volume Controller nodes can be configured to use up to three quorum disks, only one quorum disk can be elected to resolve a situation where the cluster is partitioned into two sets of nodes of equal size. The purpose of the other quorum disks is to provide redundancy if a quorum disk fails before the cluster is partitioned.
In Figure 1, the storage system that hosts the quorum disks is attached directly to a switch at both the primary and secondary sites using longwave fibre-channel connections. If either the primary site or the secondary site fails, you must ensure that the remaining site has retained direct access to the storage system that hosts the quorum disks.
An alternative configuration can use an additional fibre-channel switch at the third site with connections from that switch to the primary site and to the secondary site. This type of split-site configuration is supported only when the storage system that hosts the quorum disks supports extended quorum. Although SAN Volume Controller can use other types of storage systems for providing quorum disks, access to these quorum disks is always through a single path.
For quorum disk configuration requirements, see the Guidance for Identifying and Changing Managed Disks Assigned as Quorum Disk Candidates technote at the following Web site:
Guidance for Identifying and Changing Managed Disks Assigned as Quorum Disk Candidates