On the VIO client, DMP uses the Active/Standby array mode for the vSCSI devices. Each path to the vSCSI device is through a VIO server. One VIO server is Active and the other VIO servers are Standby. An Active/Standby array permits I/O through a single Active path, and keeps the other paths on standby. During failover, I/O is scheduled on one of the standby paths. After failback, I/Os are scheduled back onto the original Active path. The Active/Standby mode is a variation of an active/active array; only one path is active at a time.
# vxdmpadm listenclosure all ENCLR_NAME ENCLR_TYPE ENCLR_SNO STATUS ARRAY_TYPE LUN_COUNT =============================================================== ibm_vscsi0 IBM_VSCSI VSCSI CONNECTED VSCSI 9
# vxdmpadm getattr enclosure ibm_vscsi0 iopolicy ENCLR_NAME DEFAULT CURRENT ============================================ ibm_vscsi0 Single-Active Single-Active
For vSCSI devices, DMP balances the load between the VIO servers, instead of balancing the I/O on paths. By default, the iopolicy attribute of the vSCSI array is set to lunbalance. When lunbalance is set, the vSCSI LUNs are distributed so that the I/O load is shared across the VIO servers. For example, if you have 10 LUNs and 2 VIO servers, 5 of them are configured so that VIO Server 1 is Active and VIO Server 2 is Standby. The other 5 are configured so that the VIO Server 2 is Active and VIO Server 1 is Standby. To turn off load sharing across VIO servers, set the iopolicy attribute to nolunbalance.
DMP dynamically balances the I/O load across LUNs. When you add or remove disks or paths in the VIO client, the load is rebalanced. Temporary failures like enabling or disabling paths or controllers do not cause the I/O load across LUNs to be rebalanced.