VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 3, Administering Dynamic Multipathing (DMP)
DMP in a Clustered Environment
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For active/active type disk arrays, any disk can be simultaneously accessed through all
available physical paths to it. Therefore, in a clustered environment all hosts do not need
to access a disk, via the same physical path.
Note If the vxdctl enable command is run, and DMP identifies a disabled primary
path of ashareddisk in an active/passive type disk array as physically accessible,it
marks this path as enabled. However, I/O continues to use the current path and is
not routed through the path that has been marked enabled. This behavior on
clusters deviates from that on a single-host where I/O automatically fails back to
the primary path.
Enabling/Disabling Controllers with Shared Disk Groups
VxVM does not allow enabling or disabling of controllers connected to a disk that is part
of a shared VERITAS Volume Manager disk group.
For example, consider a disk array that is connected through controller c0 to a host. This
controller has a disk that is part of a shared disk group. In such a situation, the following
operations fail on that host:
# vxdmpadm disable ctlr=c0
# vxdmpadm enable ctlr=c0
The following error message is displayed:
vxvm: vxdmpadm: ERROR: operation not supported for shared disk
arrays.
Operation of the DMP Restore Daemon with Shared Disk
Groups
The DMP restore daemon does not automatically failback I/O requests for a disk in an
active/passive disk array if that disk is a part of a shared disk group.
When a restore daemon revives a DISABLED primary path to a shared disk in an
active/passive disk array, DMP doesnot routethe I/Orequestsautomaticallythroughthe
primary path, but continues routing them through the secondary path. This behavior
deviates from that in a single host environment.
In a clustered environment, failback of I/O requests to the primary path happens only
when the secondary path becomes physically inaccessible to the host.