Example iSCSI configurations

Configure your IBM Spectrum Virtualize™ in a network that uses Internet Small Computer System Interface (iSCSI).

Figure 1 shows a two-node IBM Spectrum Virtualize clustered system 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 system also acts as the system configuration node. In this example, port 1 on the configuration node provides the system management IP interface.

Figure 1. iSCSI configuration with a single subnet

Figure 2 shows a two-node IBM Spectrum Virtualize system that is connected to multiple subnets. Each node has two Ethernet ports (port 1 and port 2) that are connected to different IP subnets. One node in the system also acts as the configuration node. This configuration provides alternate IP interfaces on different subnets for the system management interface.

Note: With a multiple subnet configuration, if one subnet fails and the iSNS server is part of that subnet, the nodes and hosts are not able to access the iSNS server. The I/O is not affected and the other subnet is still active.

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

Figure 3 shows the same two-node IBM Spectrum Virtualize system as Figure 2. This figure illustrates the benefits of using multipathing and redundant host interface adapters in the host systems.

Figure 3. iSCSI configuration with host multipathing

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

Host 2 uses multipathing. A volume in the IBM Spectrum Virtualize 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 volume. The multipathing driver selects one or more of these devices during I/O. If the connection between the host and one IBM Spectrum Virtualize node port is lost, the multipathing driver can select an alternative path to the IBM Spectrum Virtualize I/O group. The I/O between the host and IBM Spectrum Virtualize continue without error. However, host 2 has only one host interface adapter. Therefore, host 2 reports I/O errors (such as the link at Y is broken) if the connection between that host interface adapter and the network is lost.

Host 3 uses multipathing and redundant host interface adapters. If a host interface adapter fails, the multipathing driver can still find paths from the host to a volume in the IBM Spectrum Virtualize I/O group, and the application I/O can continue without error. Because the host interface adapters are connected to different IP networks, the overall configuration can tolerate a single network failure without I/O errors occurring on host 3.

Multipathing drivers are not required to do system maintenance when IBM Spectrum Virtualize nodes are removed or replaced in an I/O group. However, multipathing host drivers are required for load balancing and for surviving host interface adapter, link, or network failures.