How DMP handles I/O for vSCSI devices

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How DMP handles I/O for vSCSI devices

See Setting up DMP for vSCSI devices in the Virtual I/O Client

See Setting the DMP policy for handling I/O for vSCSI devices

See Adding and removing DMP support for vSCSI devices for an array

See Using Storage Foundation in the VIO client with virtual SCSI devices

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.

The DMP I/O policy for vSCSI enclosures is always Single-Active. It is not possible to change the DMP I/O policy for the vSCSI enclosure, because only one VIO server can be Active.

The following command shows the vSCSI enclosure:

# vxdmpadm listenclosure all
ibm_vscsi0 IBM_VSCSI  VSCSI     CONNECTED VSCSI       9

The following command shows the I/O policy for the vSCSI enclosure:

# vxdmpadm getattr enclosure ibm_vscsi0 iopolicy
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.

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