Multipath I/O for integrated servers

Multipath I/O enables multiple storage connections and provides automatic failover between connections to ensure that storage is accessible in case of a hardware failure.

A single IBM® i iSCSI target port can support several servers or hosted systems.

iSCSI initiator port connection capabilities vary by operating system type. For details, see Configuring multipath I/O for integrated servers in the IBM i iSCSI Solution Guide Link outside the Information Center .

You can configure the iSCSI environment to support multiple iSCSI targets, multiple iSCSI initiators, and multiple storage connections.

This image shows the network connections between IBM i and a hosted system that has multiple iSCSI adapters installed. — Figure 1. An environment with multiple iSCSI adapters installed in the target and initiator systems

Paths

Paths are connection points between virtual devices and iSCSI target adapters in IBM i. A virtual device being hosted by IBM i is said to be linked to a path. iSCSI initiator adapters access the virtual device through the path.

IBM i virtual storage or devices are linked to a network server host adapter (NWSH) object. For example, a configured virtual disk (such as Drive C:) hosted in IBM i is linked to the NWSH that represents the iSCSI target adapter.

There are several storage paths defined in Figure 1. The paths labeled 1 and 2 each represent a single iSCSI target adapter. The path labeled M represents the multipath group, which is group of iSCSI target adapters.

You can configure storage for iSCSI-attached servers to use either a single path or a multipath group.

Removable media and virtual Ethernet connections use a single path. Connections for these devices cannot use the multipath group.

Multipath I/O and storage connection redundancy

A hosted system can use multiple iSCSI data paths to access virtual disks hosted by IBM i.

You can configure a multipath group of two or more iSCSI target adapters. Then specify that a virtual disk is accessed using the multipath group instead of a single iSCSI target adapter. With this configuration, the data on the virtual disk can be accessed using any of the iSCSI target adapters in the multipath group.

In Figure 1, the multipath group is defined as path M. The virtual disks that are linked to the multipath group can be accessed by any of the iSCSI target adapters that are also linked to the multipath group. Only one multipath group can be defined per hosted system. This group can include up to four iSCSI target adapters.

For the most reliable storage network, do the following things:

Configure multiple iSCSI targets in IBM i and define a multipath group that contains them.
Configure multiple iSCSI initiators in the System x or blade product and configure them in the IBM i remote system configuration.
Configure multiple switches to provide redundant network connections between the iSCSI targets and iSCSI initiators.
- If you are using a BladeCenter system, configure multiple switch modules.
- If you are using System x hardware, configure multiple switches in the iSCSI network.
Link all storage to the multipath group.

Note: Removable media devices cannot use the multipath group.

The advantage of the multipath configuration is that, if there is a hardware failure, the hosted system can continue to access the disks that are configured to use the multipath group, using any of the iSCSI target adapters that are configured in the multipath group. This configuration can provide uninterrupted storage connections in case of a problem with an iSCSI target adapter, an iSCSI initiator adapter or a switch.

See Configuring multipath I/O for integrated servers in the IBM i iSCSI Solution Guide Link outside the Information Center for more information about installing the required software components and linking storage to the multipath group.

Virtual Ethernet and initiator connection redundancy

Virtual Ethernet does not have the same multipath I/O concept that storage does. Virtual Ethernet supports iSCSI initiator redundancy, but not iSCSI target redundancy:

If the integrated server has multiple iSCSI initiator adapters, the iSCSI initiator that is used for a particular virtual Ethernet adapter is automatically selected. If there are no failures, the virtual Ethernet adapter continues to use the selected iSCSI initiator. However, if the iSCSI initiator connection fails (for example, an initiator cable is pulled or the initiator card fails), a different iSCSI initiator adapter is automatically selected for the virtual Ethernet adapter and is used until another failure occurs.
Note: In order for the automated selection process to work, the configured iSCSI target adapter must still be accessible by at least one iSCSI initiator adapter that is listed in the IBM i remote system configuration.
There is no multipath group available for virtual Ethernet. A virtual Ethernet adapter is configured to use a specific iSCSI target and always uses that target. If the iSCSI target adapter fails or its cable is pulled, any virtual Ethernet adapters that are configured to use that iSCSI target adapter stop communicating. However, if the cable is plugged back in, communication automatically resumes.

For the most reliable virtual Ethernet network, do the following things:

Configure multiple iSCSI initiators in the System x or blade product and configure them in the IBM i remote system configuration.
Ensure that multiple iSCSI initiators can access the same IBM i iSCSI target.