With iSCSI storage, you can take advantage of the multipathing support that the IP network offers. In addition, ESX supports host-based multipathing for all types of iSCSI initiators.

ESX can use multipathing support built into the IP network, which allows the network to perform routing. Through dynamic discovery, iSCSI initiators obtain a list of target addresses that the initiators can use as multiple paths to iSCSI LUNs for failover purposes.

ESX also supports host-based multipathing.

Host-Based Multipathing shows multipathing setups possible with different types of iSCSI initiators.

Host-Based Multipathing
The graphic shows multipathing setups possible with different types of iSCSI initiators.

With the hardware iSCSI, the host typically has two or more hardware iSCSI adapters available, from which the storage system can be reached using one or more switches. Alternatively, the setup might include one adapter and two storage processors so that the adapter can use a different path to reach the storage system.

In the Host-Based Multipathing illustration, Host1 has two hardware iSCSI adapters, HBA1 and HBA2, that provide two physical paths to the storage system. Multipathing plug-ins on your host, whether the VMkernel NMP or any third-party MPPs, have access to the paths by default and can monitor health of each physical path. If, for example, HBA1 or the link between HBA1 and the network fails, the multipathing plug-ins can switch the path over to HBA2.

With the software iSCSI, as shown on Host 2 of Host-Based Multipathing, you can use multiple NICs that provide failover and load balancing capabilities for iSCSI connections between your host and storage systems.

For this setup, because multipathing plug-ins do not have direct access to physical NICs on your host, you must connect each physical NIC to a separate VMkernel port. You then associate all VMkernel ports with the software iSCSI initiator using a port binding technique. As a result, each VMkernel port connected to a separate NIC becomes a different path that the iSCSI storage stack and its storage-aware multipathing plug-ins can use.

For information about how to configure multipathing for the software iSCSI, see Networking Configuration for Software iSCSI and Dependent Hardware iSCSI.