In addition to traditional iSCSI, ESXi supports the iSCSI Extensions for RDMA (iSER) protocol. When the iSER protocol is enabled, the iSCSI framework on the ESXi host can use the Remote Direct Memory Access (RDMA) transport instead of TCP/IP.
The traditional iSCSI protocol carries SCSI commands over a TCP/IP network between an iSCSI initiator on a host and an iSCSI target on a storage device. The iSCSI protocol encapsulates the commands and assembles that data in packets for the TCP/IP layer. When the data arrives, the iSCSI protocol disassembles the TCP/IP packets, so that the SCSI commands can be differentiated and delivered to the storage device.
iSER differs from traditional iSCSI as it replaces the TCP/IP data transfer model with the Remote Direct Memory Access (RDMA) transport. Using the direct data placement technology of the RDMA, the iSER protocol can transfer data directly between the memory buffers of the ESXi host and storage devices. This method eliminates unnecessary TCP/IP processing and data coping, and can also reduce latency and the CPU load on the storage device.
In the iSER environment, iSCSI works exactly as before, but uses an underlying RDMA fabric interface instead of the TCP/IP-based interface.
Because the iSER protocol preserves the compatibility with iSCSI infrastructure, the process of enabling iSER on the ESXi host is similar to the iSCSI process. See Configure iSER with ESXi.