132Administering dynamic multipathing (DMP)

How DMP works

and under the new naming scheme as:

#vxdisk list

DEVICE

TYPE

DISK

GROUP

STATUS

disk155

auto:LVM

-

-

LVM

disk156

auto:LVM

-

-

LVM

disk224

auto:cdsdisk

-

-

online

disk225

auto:cdsdisk

-

-

online

disk226

auto:cdsdisk

-

-

online

disk227

auto:cdsdisk

-

-

online

disk228

auto:cdsdisk

-

-

online

disk229

auto:cdsdisk

-

-

online

See “Changing the disk-naming scheme” on page 91.

DMP in a clustered environment

Note: You need an additional license to use the cluster feature of VxVM.

In a clustered environment where Active/Passive type disk arrays are shared by multiple hosts, all nodes in the cluster must access the disk via the same physical path. Accessing a disk via multiple paths simultaneously can severely degrade I/O performance (sometimes referred to as the ping-pong effect). Path failover on a single cluster node is also coordinated across the cluster so that all the nodes continue to share the same physical path.

Prior to release 4.1 of VxVM, the clustering and DMP features could not handle automatic failback in A/P arrays when a path was restored, and did not support failback for explicit failover mode arrays. Failback could only be implemented manually by running the vxdctl enable command on each cluster node after the path failure had been corrected. In release 4.1, failback is now an automatic cluster-wide operation that is coordinated by the master node. Automatic failback in explicit failover mode arrays is also handled by issuing the appropriate low-level command. If required, this feature can be disabled by selecting the “no failback” option that is defined in the array policy module (APM) for an array.

Note: Support for automatic failback of an A/P array requires that an appropriate ASL (and APM, if required) is available for the array, and has been installed on the system. See “Administering the Device Discovery Layer” on page 85 and “Configuring array policy modules” on page 162.

For Active/Active type disk arrays, any disk can be simultaneously accessed through all available physical paths to it. In a clustered environment, the nodes do not all need to access a disk via the same physical path.