You must follow the SAN Volume Controller configuration rules for fibre-channel switches to ensure that you have a valid configuration.
The SAN must contain only supported switches.
See the following Web site for specific firmware levels and the latest supported hardware:
www.ibm.com/storage/support/2145
Configuring your SAN with at least two independent switches, or networks of switches, ensures a redundant fabric with no single point of failure. If one of the two SAN fabrics fails, the configuration is in a degraded mode, but is still valid. A SAN with only one fabric is a valid configuration but risks loss of access to data if the fabric fails. SANs with only one fabric are exposed to a single point of failure.
Configurations with more than four SANs are not supported.
For fibre-channel connections, the SAN Volume Controller nodes must always be connected to SAN switches only. Each node must be connected to each of the counterpart SANs that are in the redundant fabric. Any fibre-channel configuration that uses a direct physical connection between a host and a SAN Volume Controller node is not supported. When attaching iSCSI hosts to SAN Volume Controller nodes, Ethernet switches must be used.
All backend storage systems must always be connected to SAN switches only. Multiple connections are permitted from redundant storage systems to improve data bandwidth performance. A connection between each redundant storage system and each counterpart SAN is not required. For example, in an IBM System Storage DS4000® configuration in which the IBM® DS4000 contains two redundant storage systems, only two storage system minihubs are usually used. Storage system A is connected to counterpart SAN A, and storage system B is connected to counterpart SAN B. Any configuration that uses a direct physical connection between the SAN Volume Controller node and the storage system is not supported.
When you attach a node to a SAN fabric that contains core directors and edge switches, connect the node ports to the core directors and connect the host ports to the edge switches. In this type of fabric, the next priority for connection to the core directors is the storage systems, leaving the host ports connected to the edge switches.
A SAN Volume Controller SAN must follow all switch manufacturer configuration rules, which might place restrictions on the configuration. Any configuration that does not follow switch manufacturer configuration rules is not supported.
Mixing manufacturer switches in a single SAN fabric
Within an individual SAN fabric, only mix switches from different vendors if the configuration is supported by the switch vendors.
Fibre-channel switches and interswitch links
The SAN Volume Controller supports distance-extender technology, including DWDM (dense wavelength division multiplexing) and FCIP (Fibre Channel over IP) extenders, to increase the overall distance between local and remote clusters. If this extender technology involves a protocol conversion, the local and remote fabrics are regarded as independent fabrics, limited to three ISL hops each.
With ISLs between nodes in the same cluster, the ISLs are considered a single point of failure. This is illustrated in Figure 1.
If Link 1 or Link 2 fails, the cluster communication does not fail.
If Link 3 or Link 4 fails, the cluster communication does not fail.
If ISL 1 or ISL 2 fails, the communication between Node A and Node B fails for a period of time, and the node is not recognized, even though there is still a connection between the nodes.
To ensure that a fibre-channel link failure does not cause nodes to fail when there are ISLs between nodes, it is necessary to use a redundant configuration. This is illustrated in Figure 2.
With a redundant configuration, if any one of the links fails, communication on the cluster does not fail.
ISL oversubscription
Perform a thorough SAN design analysis to avoid ISL congestion. Do not configure the SAN to use SAN Volume Controller to SAN Volume Controller traffic or SAN Volume Controller to storage system traffic across ISLs that are oversubscribed. For host to SAN Volume Controller traffic, do not use an ISL oversubscription ratio that is greater than 7 to 1. Congestion on the ISLs can result in severe SAN Volume Controller performance degradation and I/O errors on the host.
SAN Volume Controller in a SAN with director class switches
You can use director class switches within the SAN to connect large numbers of RAID controllers and hosts to a SAN Volume Controller cluster. Because director class switches provide internal redundancy, one director class switch can replace a SAN that uses multiple switches. However, the director class switch provides only network redundancy; it does not protect against physical damage (for example, flood or fire), which might destroy the entire function. A tiered network of smaller switches or a core-edge topology with multiple switches in the core can provide comprehensive redundancy and more protection against physical damage for a network in a wide area. Do not use a single director class switch to provide more than one counterpart SAN because this does not constitute true redundancy.