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Introduction to Microsoft Multipath I/O
6 Dell EMC SC Series Storage and Microsoft Multipath I/O | CML1004
2 Introduction to Microsoft Multipath I/O
Microsoft MPIO is a framework that allows administrators to configure load balancing and failover processes
for Fibre Channel (FC), iSCSI, and serial attached SCSI (SAS) front-end (FE) connected storage devices.
Load balancing can be configured to use up to 32 independent paths for each connected storage device.
Two to four paths per storage device is a common configuration with SC Series storage.
SC Series arrays provide native redundancy and failover protection with multiple controllers and RAID modes.
From the perspective of the host server, path redundancy (for both load balancing and failover) is provided by
MPIO.
The MPIO framework uses a device-specific module (DSM) which is software that allows the host server to
recognize and intelligently manage multiple paths to the same SAN volume. Without a DSM, the host is
unable to manage multiple paths and as a result, Disk Management erroneously reports multiple instances of
the same disk device, one disk for each path. Microsoft provides a built-in DSM (MSDSM) for Windows
Server 2008 R2 and above that is fully compatible with SC Series storage, and is the focus of this paper.
2.1 SC Series storage front-end connection options
Several MPIO-capable front-end connection cabling options are available with SC Series storage. Regardless
of the type of connection used, with MPIO, the host server will see multiple paths to storage objects when
multiple paths are presented to the host server.
This section provides an overview of these connection options to aid with understanding the overall SC Series
MPIO architecture, but it does not provide detailed front-end connection configuration guidance. For front-end
configuration guidance, including detailed cabling examples, see the storage system configuration and
deployment guide or owner’s manual for your SC Series array on the Dell EMC SC Series Storage product
page.
2.1.1 Legacy port mode
In legacy port mode, front-end I/O ports are assigned to fault domains as either primary or reserve ports. I/O
uses the primary ports only. Reserve ports stay in standby mode. If a primary port fails, I/O will fail over to the
reserve port. Legacy port mode requires twice as many I/O ports as virtual port mode in order to enable
multiple paths and therefore makes less efficient use of the available hardware. Legacy port mode, as the
name suggests, is an older (but still supported) configuration option that is not recommended unless a
specific workload or operating system requires it. For example, some non-Microsoft operating systems may
require legacy port mode.