Example iSCSI configurations

These examples show typical ways to configure your SAN Volume Controller in a network that uses iSCSI.

Figure 1 shows a four-node SAN Volume Controller cluster that is connected to a single subnet. Each node has two Ethernet ports, each of which is used for iSCSI data transfers. One node in the cluster also acts as the cluster configuration node. In this example, port 1 on the configuration node provides the cluster-management IP interface.

Figure 1. iSCSI configuration with a single subnet

Figure 2 shows a two-node SAN Volume Controller cluster that is connected to multiple subnets. Each node has two Ethernet ports (port 1 and port 2) that are connected to different IP subnets. In addition, one node in the cluster also acts as the cluster configuration node, which provides alternate IP interfaces, again on different subnets for the cluster management interface.

Figure 2. iSCSI configuration that uses multiple subnets and provides alternate configuration interfaces

Figure 3 shows the same two-node SAN Volume Controller cluster as Figure 2. This figure illustrates the benefits of using multipathing and redundant NICs in the host systems.

Figure 3. iSCSI configuration with host multipathing

In this example, host 1 does not use multipathing. A VDisk in the SAN Volume Controller I/O group appears as four separate devices in host 2. The host selects one device to perform I/O to the VDisk, which corresponds to a particular IP address at a SAN Volume Controller node port, 10.10.1.10. If a connection between the host and this SAN Volume Controller port is broken (the link at X is broken), an I/O error is recorded on host 1 for that VDisk if I/O is in progress. No SAN Volume Controller state changes or IP failover occur.

Host 2 uses multipathing. A VDisk in the SAN Volume Controller I/O group appears as a single device to the applications on host 2, even though the multipathing driver can detect four separate devices for each VDisk. The multipathing driver selects one or more of these devices during I/O. If the connection between the host and one SAN Volume Controller node port is lost, the multipathing driver can select an alternative path to the SAN Volume Controller I/O group. The I/O between the host and SAN Volume Controller continue without error. Host 2, however, has only one NIC and will therefore report I/O errors (such as the link at Y is broken) if the connection between that NIC and the network is lost.

Host 3 uses multipathing and redundant NICs. This means that if an NIC fails, the multipathing driver can still find paths from the host to a VDisk in the SAN Volume Controller I/O group and the application I/O can continue without error. Because the NICs are connected to different IP networks, the overall configuration can tolerate a single network failing without I/O errors occurring on host 3.

Multipathing drivers are not required to do cluster maintenance when SAN Volume Controller nodes are removed or replaced in an I/O group. However, multipathing host drivers are required for load balancing and for surviving NIC, link, or network failures.