SDD dynamic pathing on hosts running the Windows NT operating system

The subsystem device driver (SDD) for Windows® supports dynamic pathing for hosts that run the Windows NT® operating system.

SDD supports dynamic pathing when you add more paths to an existing VDisk and when you present a new VDisk to the host. No user intervention is required, other than is normal for a new device discovery under Windows operating systems.

Preferred paths are also supported with SDD for Windows. When you use clustering, SDD is aware of the preferred paths that the SAN Volume Controller sets for each VDisk. In this case, SDD uses its reserve policy to reserve a single path to the device and uses a preferred path if one is available. If you do not use clustering, SDD uses its load-balancing policy that tries to equalize the load across all preferred paths. If preferred paths are available, SDD uses the path that has the least I/O at the time. If SDD finds no available preferred paths, it tries to balance the load across all the paths it does find and uses the least active non-preferred path.

When you configure, keep in mind the SDD for Windows maximum configuration, which is provided in Table 1.

Table 1. Configuration maximums for SDD for Windows
Object	SDD maximum	Description
VDisk	512 (See Note 1.)	The maximum number of VDisks that can be supported by the SAN Volume Controller for a host that runs a Microsoft® Windows operating system (per host object).
Paths per VDisk (See Note 2.)	8	The maximum number of paths to each VDisk.
Notes: You can assign a maximum of 26 individual drive letters to a host that runs the Windows NT operating system. SDD for Windows supports 16 paths per VDisk, but SAN Volume Controller supports only a maximum of eight paths to ensure a reasonable path-failover time.