White Papers

6 PS Series Architecture: MPIO with Devices That Have Unequal Link Speeds | TR1105 v1
2 Multi-Path I/O
MPIO uses redundant physical (or virtual) connections to deliver high availability to shared storage. Having
multiple host connections, switches and SAN interfaces helps to eliminate single points of failure. Using
MPIO, servers can send multiple I/O streams to SAN volumes concurrently. MPIO routes I/O over
redundant paths that connect servers to storage and manages these paths so that requests can be
rerouted through another path in the event of a failure in one of the components along the way. MPIO
also provides increased redundancy and can improve performance of application data hosted on the SAN.
MPIO implementations supplied with the Microsoft Windows, VMware or Linux operating systems are
necessarily generic in their implementation since they must work reasonably well with a wide range of
storage products. These implementations typically require that the administrator manually create
individual MPIO connections to the storage and then often uses a blind round-robin algorithm to
distribute I/O between these connections. Dell has developed extensions to these generic MPIO
implementations for the PS Series arrays that automates the establishment of the MPIO connections,
enables the built in multi-path modules of these operations systems to obtain specific information about
their PS Series storage and use a Least Queue Depth algorithm to select the port for communicating with
the storage. This combination of features results in more efficient and intelligent use and connectivity to
the SAN hardware.
2.1 Dell PS Series MPIO extensions
The Dell PS Series Multi-Path I/O extensions are designed to deliver:
 Automatic connection management
 Automatic path failure detection and path failover
 Automatic load balancing across multiple paths
 Support for multiple connections to a single iSCSI target (volume)
 Increased bandwidth
 Reduced network latency
 Easy installation and management
 Automatically throttle down connections when a storage pool reaches 90% of maximum
allowable connections
The Dell PS Series MPIO extensions consist of two components:
 A driver component in the I/O Stack that works in conjunction with the native MPIO driver to route
I/O to the desired path
For Microsoft Windows this is referred to as the Device Specific Module (DSM) and for VMware the
term used is the Multipathing Extension Module (MEM). Linux has no standard term. This
component selects the best path for each I/O by using knowledge about how the volume is laid
out on the PS Series Group.
 A service or daemon, (EqualLogic Host Connection Manager – EHCM) that manages connections