A DSM is a Device Specific Module. It is the “personality” module that is loaded by the MS MPIO driver to handle customer failover calls. It is provided by a third party vendor (for the DS400 it is QLogic) to implement the actual movement of devices. The DSM module is provided to differentiate each storage device.
Multiple DSMs can coexist but only one instance of MPIO is required. Each DSM interfaces directly to the hardware and should only claim its storage device. The DSM is dependent on the MS MPIO driver which must be installed as well. The DSM installer installs three MS drivers in addition to the DSM (qldsm.sys). These are mpdev.sys, mpio.sys and mpfiltr.sys.
The MPIO driver is the failover mechanism that builds and manages the path definitions. Problems associated with a DSM device are reported in the NT event log from either the Qlogic DSM (qldsm) or the multi-path driver (MPIO). Problems associated with path corruption should be investigated. This means that a device is either not present on a particular path or the path provided is NULL.
The current failover mechanism used by the DSM is the SetTargetPortGroup command.