When preparing your ESXi system to use iSCSI SAN storage you need to set LUN allocations.
Note the following points:
Storage Provisioning. To ensure that the host recognizes LUNs at startup time, configure all iSCSI storage targets so that your host can access them and use them. Also, configure your host so that it can discover all available iSCSI targets.
vMotion and VMware DRS. When you use vCenter Server and vMotion or DRS, make sure that the LUNs for the virtual machines are provisioned to all hosts. This configuration provides the greatest freedom in moving virtual machines.
Active-active versus active-passive arrays. When you use vMotion or DRS with an active-passive SAN storage device, make sure that all hosts have consistent paths to all storage processors. Not doing so can cause path thrashing when a vMotion migration occurs.
For active-passive storage arrays not listed in Storage/SAN Compatibility, VMware does not support storage-port failover. You must connect the server to the active port on the storage system. This configuration ensures that the LUNs are presented to the host.