Improve fault tolerance in cluster server for iSCSI

An iSCSI cluster server has a complex networking structure because each cluster uses no less than three networks to tie everything together.

The three networks are:

The iSCSI SAN. This should be on a physically independent network, and the network interface card (NIC) should not be enabled for cluster use.

A public network. This allows the cluster to communicate with external clients.

A private network. This network communicates among the clusters and must be a physically separate network.

Each node in the cluster needs a separate network interface card to support each network.

NIC teaming, an obvious way to improve fault tolerance in a highly networked environment like this, binds two physical NICs together into a single logical NIC and eliminates a single point of failure in the network.

However, Cluster Server for iSCSI only supports NIC teaming on the public network. It does not support it on a storage area network or private network. To get more fault tolerance on the SAN in an iSCSI cluster server, Microsoft recommends using either MS MPIO (multipath I/O) or Multiple Connections per Session (MCS from the iSCSI specification) on the SAN.

Both MCS and MPIO allow failover among multiple NICs and both of them can support I/O load balancing as well. MCS offers multiple TCP/IP connections between the server and the storage device during the same iSCSI session. These can be on different physical links as well. MPIO lets the

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server log multiple sessions to the storage array.

Microsoft discusses this in a TechNet article, "Server Clusters Network Configuration Best Practices for Windows 2000 and Windows Server 2003" located here: www.microsoft.com/technet/prodtechnol/windowsserver2003/technologies/clustering/clstntbp.mspx.

Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.

This was first published in October 2005

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