Windows Server 2008 iSCSI multiple connected session modes

For administrators using Windows Server 2008’s iSCSI initiator, selecting the right connection policy is critical. Rick Vanover explains the options for the ease-of-entry storage protocol.

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Adding an iSCSI volume is straightforward on Windows Server 2008 R2, yet the policy selection may be omitted from the configuration. Windows Server 2008 R2 manages the access policy by the multiple connected session (MCS) policy configuration. For system administrators with fibre channel storage and multipathing policies, this terminology will make sense when applied to iSCSI volumes.

Once an iSCSI volume is added to Windows, it may be necessary to review the MCS configuration. The default MCS policy is round robin for iSCSI targets. The available configuration policies are:

• Round robin: This policy attempts to evenly distribute storage traffic to all available processing paths.
• Fail over only: This policy designates one active (primary) path to the storage resource; the remaining paths are assigned to a standby role. If the remaining paths are engaged for active storage, they will attempt to come online with a round robin policy until one is available.
• Round robin with subset: This policy runs round robin on active paths and has standby paths that are available via round-robin if the active paths fail.
• Least queue depth: This policy attempts to distribute loads based on I/O requests and the associated queue length. The round robin policy simply processes each request equally, with no regard to the load associated with the I/O task on the path.
• Weighted paths: This policy designates a path to have a low number (priority) to a path.

Configuring the MCS policy is a result of thorough network planning for the iSCSI networks and the associated resources. A typical configuration is a number of interfaces on a server going to different switches that are made available to the iSCSI target. Most configurations will have two interfaces designated to process the iSCSI storage. Using the MCS policies can allow additional interfaces to act as a failover path to the storage. An example would be the operating system interface designated as a failover interface for the iSCSI storage, but only as a contingency because of traffic prioritization.

Have you used non-default MCS policies? Share your experiences in the forums.

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