The Internet Small Computer Systems Interface (iSCSI) is a protocol used to transport SCSI commands over a TCP/IP network. In an iSCSI environment, the client machine (in this case, an ESXi host) uses iSCSI initiators (hardware/software) to connect to iSCSI targets on an iSCSI storage system.
Let's review some of the iSCSI terminology that you will need to be aware of before you learn how to configure and manage access to iSCSI storage:
- iSCSI initiator: This is a software/hardware adapter that resides on an ESXi host and has the ability to connect to an iSCSI target. The software iSCSI adapter is built into the VMkernel and can be enabled when intended. The hardware iSCSI adapter can be of two types: dependent and independent. While the dependent iSCSI adapter handles the packet processing, it is still reliant on ESXi for its network configuration and management. The independent iSCSI adapter provides for both configuration and packet processing.
- iSCSI target: This is a network interface on the iSCSI array or on a LUN. Some arrays, such as Dell EqualLogic and HP StoreVirual, present each LUN as a target.
With vSphere 6.5, unlike the previous versions, the iSCSI initiator and the iSCSI target can now be on two different layer-2 subnets.
- iSCSI portal: This is a combination of the iSCSI target's IP address and the listening port (default: 3260). An iSCSI portal at the initiator is the IP address of the VMkernel interface.
- iSCSI session: This is a TCP/IP session established between an iSCSI initiator and an iSCSI target. Each session can have one more connection to the target. In the case of software iSCSI, a session is established between each bound VMkernel interface and an iSCSI target. For example, if there are two VMkernel interfaces bound to the iSCSI initiator, then the initiator will establish two separate sessions for each target it discovers.
- iSCSI connection: Each iSCSI session can have multiple connections to the iSCSI target portal.
- CHAP: The Challenge Handshake Authentication Protocol (CHAP) is used by iSCSI to make sure that the initiator and target establish a trusted and secure connection.
- Dynamic Discovery: This is a commonly used target discovery mechanism, which comes in handy when the iSCSI server has made a large number of LUNs/targets via its target portal.
- Static Discovery: Unlike the Dynamic Discovery mechanism, static discovery does not see every LUN/target exposed via the target portal. Instead, it only sees the specified targets.