HP Veritas Volume Manager 5.0 -UX 11i v3 manual Load balancing, Throttling

Administering dynamic multipathing (DMP)

How DMP works

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DMP is also informed when a connection is repaired or restored, and when you add or remove devices after the system has been fully booted (provided that the operating system recognizes the devices correctly).

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.

For information about how to configure the behavior of DMP in response to I/O failure on a path, see “Configuring the response to I/O failures” on page 156.

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.

For information about how to configure I/O throttling on a path, see “Configuring the I/O throttling mechanism” on page 157.

Load balancing

By default, DMP uses a minimum queue length policy (minimumq) to provide load balancing across paths for Active/Active disk arrays. I/O is sent down the path that has the minimum number of outstanding I/O requests in the queue for a LUN.

For Active/Passive and Asymmetric Active/Active disk arrays, the default round-robinpolicy shares I/O equally between the available paths in a round-robin sequence. If the primary path fails, I/O is switched over to another available primary or secondary path. 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.

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 147.