VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Administering Dynamic Multipathing (DMP)
DMP in a Clustered Environment
Chapter 3 129
DMP in a Clustered Environment
NOTE You may need an additional license to use this feature.
In a clustered environment where active/passive type disk arrays are
shared by multiple hosts, all hosts in the cluster should access the disk
via the same physical path. If a disk from an active/passive type shared
disk array is accessed via multiple paths simultaneously, it could lead to
severe degradation of I/O performance. This requires path failover on a
host to be a cluster coordinated activity for an active/passive type disk
array.
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 a shared disk 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 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