Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Load balancing
By default, the DMP uses the Minimum Queue policy for load balancing across
paths for Active/Active (A/A), Active/Passive (A/P), Active/Passive with explicit
failover (A/P-F) and Active/Passive with group failover (A/P-G) disk arrays. Load
balancing maximizes I/O throughput by using the total bandwidth of all available
paths. I/O is sent down the path which has the minimum outstanding I/Os.
For A/P disk arrays, I/O is sent down the primary paths. If the primary paths fail,
I/O is switched over to the available secondary paths. As the continuous transfer
of ownership of LUNs from one controller to another results in severe I/O
slowdown, load balancing across primary and secondary paths is not performed
for A/P disk arrays unless they support concurrent I/O.
For A/P, A/P-F and A/P-G arrays, load balancing is performed across all the
currently active paths as is done for A/A arrays.
You can use the vxdmpadm command to change the I/O policy for the paths to an
enclosure or disk array.
See “Specifying the I/O policy” on page 180.
DMP coexistence with HP-UX native multi-pathing
The HP-UX 11i v3 release includes support for native multi-pathing, which can
coexist with DMP. HP-UX native multi-pathing creates a persistent (agile) device
in the /dev/disk and /dev/rdisk directories for each disk that can be accessed
by one or more physical paths. To maintain backward compatibility, HP-UX also
creates legacy devices in the /dev/dsk and /dev/rdsk directories.
DMP recreates disk devices for all paths in the operating system’s hardware device
tree as DMP nodes in the /dev/vx/dmp and /dev/vx/rdmp directories,
independently of the devices that are listed in the /dev/dsk and /dev/rdsk
directories. A DMP node represents a disk that can be accessed by one or more
physical paths. DMP nodes are not used by the native multi-pathing feature of
HP-UX.
DMP commands display device names according the naming scheme that has
been selected.
See “Changing the disk-naming scheme” on page 99.
See “Foreign devices” on page 97.
See “Migrating between DMP and HP-UX native multi-pathing” on page 148.
For more information on administering native multi-pathing with Base-VxVM and
VxVM-Full, see the Veritas Volume Manager Release Notes.
147Administering Dynamic Multi-Pathing
How DMP works