If you are an administrator who plans to set up ESXi hosts to work with iSCSI SANs, you must have a working knowledge of iSCSI concepts.
iSCSI SANs use Ethernet connections between computer systems, or host servers, and high-performance storage subsystems. On the host side, the SAN components include iSCSI host bus adapters (HBAs) or Network Interface Cards (NICs). They also include switches and routers that transport the storage traffic, cables, storage processors (SPs), and storage disk systems.
iSCSI SAN uses a client-server architecture. The client, called iSCSI initiator, operates on your host. It initiates iSCSI sessions by issuing SCSI commands and transmitting them, encapsulated into iSCSI protocol, to a server. The server is known as an iSCSI target. The iSCSI target represents a physical storage system on the network. The target can also be a virtual iSCSI SAN, for example, an iSCSI target emulator running in a virtual machine. The iSCSI target responds to the initiator's commands by transmitting required iSCSI data.
When transferring data between the host server and storage, the SAN uses a technique known as multipathing. With multipathing, your ESXi host can have more than one physical path to a LUN on a storage system.
Generally, a single path from a host to a LUN consists of an iSCSI adapter or NIC, switch ports, connecting cables, and the storage controller port. If any component of the path fails, the host selects another available path for I/O. The process of detecting a failed path and switching to another is called path failover.
For more information on multipathing, see Understanding Multipathing and Failover.