iSCSI initiators are used by ESXi hosts to access iSCSI targets. iSCSI initiators encapsulate SCSI commands into Ethernet packets, enabling ESXi hosts to communicate with an iSCSI SAN device over the standard Ethernet cabling.
Two types of initiators are supported by ESXi hosts:
1. software iSCSI initiator – the initiator code that is built into the VMkernel. The iSCSI SAN device can be accessed using standard network adapters. With the software iSCSI initiator, you don’t need to purchase a hardware iSCSI adapter to obtain iSCSI connectivity, but all iSCSI processing is done by the ESXi host.
2. hardware iSCSI initiator – a specialized adapter capable of accessing an iSCSI SAN device over the standard network. All iSCSI processing is done by the adapter. Hardware iSCSI initiators are divided into two categories:
- depended hardware iSCSI adapter – an adapter that performs the iSCSI processing but relies on the VMkernel for networking access.
- independed hardware iSCSI adapter – an adapter that performs both the iSCSI processing and networking. The adapter of this type implements its own interfaces for networking, configuration, and management and does not depend on VMkernel.
The following picture illustrates the differences between the two iSCSI initiator types (source: VMware):