Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

If required, the response of DMP to I/O failure on a path can be tuned for the paths
to individual arrays. DMP can be configured to time out an I/O request either after
a given period of time has elapsed without the request succeeding, or after a given
number of retries on a path have failed.
See “Configuring the response to I/O failures” on page 185.
I/O throttling
If I/O throttling is enabled, and the number of outstanding I/O requests builds up
on a path that has become less responsive, DMP can be configured to prevent new
I/O requests being sent on the path either when the number of outstanding I/O
requests has reached a given value, or a given time has elapsed since the last
successful I/O request on the path. While throttling is applied to a path, the
outstanding I/O requests on that path are scheduled on other available paths. The
throttling is removed from the path if the HBA reports no error on the path, or if
an outstanding I/O request on the path succeeds.
See “Configuring the I/O throttling mechanism” on page 186.
Load balancing
By default, the DMP uses the Minimum Queue policy for load balancing across
paths for Active/Active, A/P-C, A/PF-C and A/PG-C 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 Active/Passive disk arrays, I/O is sent down the primary path. If the primary
path fails, I/O is switched over to the other available primary paths or secondary
paths. As the continuous transfer of ownership of LUNs from one controller to
another results in severe I/O slowdown, load balancing across paths is not
performed for Active/Passive disk arrays unless they support concurrent I/O.
Both paths of an Active/Passive array are not considered to be on different
controllers when mirroring across controllers (for example, when creating a
volume using vxassist make specified with the mirror=ctlr attribute).
For A/P-C, A/PF-C and A/PG-C arrays, load balancing is performed across all the
currently active paths as is done for Active/Active 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 175.
Administering Dynamic Multipathing
How DMP works
142