Q: What policies configure iSCSI Multipath I/O (MPIO) behavior?

A: MPIO is commonly used to aggregate multiple iSCSI network adapters. This aggregation can be used for failover or for load balancing traffic between a server and its storage. You can configure an MPIO-enabled LUN in Windows Server 2008 R2 with one of six different policies:

  • Failover only is used to provide additional availability should a connection fail, but does not provide load balancing or network aggregation.
  • Round robin is the default setting for most connections. It aggregates network connections, sending data equally across each connection.
  • Round robin with a subset of paths adds one or more additional failover adapters, which are only used when each member of the round robin group has failed.
  • Least queue depth also provides for network aggregation, but sends each I/O request through the connection with the least number of requests in queue.
  • Weighted paths provides network aggregation, but adds the ability to manually assign weights to different paths. These weights determine how heavily each connection is used in comparison with the others.
  • Least blocks is similar to least queue depth in that it unevenly distributes I/O requests; however this policy sends each I/O request through the connection with the least number of pending I/O blocks.

Prior to setting any of these policies, ensure that your storage hardware supports their use.

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