Veritas Dynamic Multipathing (DMP)

What is Dynamic Multipathing (DMP)?
DMP is a method of providing two or more hardware paths to a single drive. For example, the physical hardware can have at least two paths, such as c1t1d0 and c2t1d0, directing I/O to the same drive. VxVM arbitrarily selects one of the two names and creates a single device entry, then transfers data across both paths to spread the I/O, DMP is enabled by default. Volume Manager detects multipath systems with the Universal Worldwide Device Identifiers(WWD IDs) and manages multipath targets, such as disk arrays, which define policies for using more than one path.
Benefits of DMP

• High availability - DMP provides greater reliability through a path failover mechanism. In the event of a loss of one connection to a disk, the system continues to access the critical data over the other sound connections until the failed path is replaced.
• Improved performance - DMP provides greater I/O throughput by balancing the I/O load uniformly across multiple I/O paths to the disk device.

The disk paths can be in a active/active (use both paths) or active/passive state (use one path).

list controllers	vxdmpadm listctlr all
list enclosures	vxdmpadm listenclosure all vxdmpadm listenclosure <enclosure>
Display Paths	vxdmpadm getsubpaths ctlr=fscsi2 vxdmpadm getsubpaths dmpnodename=<enclosure>
Who controls the path	vxdmpadm getdmpnode nodename=c3t2d0 vxdmpadm getdmpnode enclosure=<enclosure>
Controller (enable/disable)	vxdmpadm disable ctlr=<controller> vxdmpadm enable ctlr=<controller>
Statistics	vxdmpadm iostat start vxdmpadm iostat reset vxdmpadm iostat show all
I/O balancing & load	vxdmpadm getattr enclosure <enclosure> iopolicy vxdmpadm setattr enclosure <enclosure> iopolicy=<policy> adaptive priority balanced (default) round-robin minimumq single-active
Path Type	vxdmpadm setattr path <path-name> pathtype=<path-type> active nomanual nopreffered primary secondary standby
Displays the restore/error daemon	vxdmpadm stat restored vxdmpadm stat errord
Starts the restore daemon	vxdmpadm start restored
Stops the restore daemon	vxdmpadm stop restored