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ISCSI Extensions for RDMA

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The iSCSI Extensions for RDMA (iSER) protocol allows the Internet Small Computer System Interface (iSCSI) protocol to use a network that provides Remote Direct Memory Access (RDMA). Typically this network is either the Transmission Control Protocol (TCP) with RDMA services (iWARP) or InfiniBand. It permits data to be transferred directly into SCSI I/O buffers without intermediate data copies.

Description

The Datamover Architecture (DA) defines an abstract model in which the movement of data between iSCSI end nodes is logically separated from the rest of the iSCSI protocol. iSER is one Datamover protocol. The interface between the iSCSI and a Datamover protocol, iSER in this case, is called Datamover Interface (DI).

The motivation for iSER is to use RDMA to avoid unnecessary data copying on the target and initiator. The main difference between the standard iSCSI and iSCSI over iSER in the execution of SCSI read/write commands is that with iSER the target drives all data transfer (with the exception of iSCSI unsolicited data) by issuing RDMA write/read operations, respectively. When the iSCSI layer issues an iSCSI command PDU, it calls the Send_Control primitive, which is part of the DI. The Send_Control primitive sends the STag with the PDU. The iSER layer in the target side notifies the target that the PDU was received with the Control_Notify primitive (which is part of the DI). The target calls the Put_Data or Get_Data primitives (which are part of the DI) to perform an RDMA write/read operation respectively. Then, the target calls the Send_Control primitive to send a response to the initiator. An example is shown in the figures (time progresses from top to bottom).

READ command execution with iSER
WRITE command execution with iSER

All iSCSI control-type PDUs contain an iSER header, which allows the initiator to advertise the STags that were generated during buffer registration. The target will use the STags later for RDMA read/write operations.

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