iSCSI overview

iSCSI is an IP-based standard for transferring data that supports host access by carrying SCSI commands over IP networks. The iSCSI standard is defined by RFC 3720.

For Lenovo Storage V7000, connections from iSCSI-attached hosts to nodes are supported. iSCSI connections from Lenovo Storage V7000 nodes to storage systems are not supported.

Table 1 shows that iSCSI and Fibre Channel terms have analogous components.

Table 1. Comparison of iSCSI and Fibre Channel components
iSCSI components	Fibre Channel components
iSCSI host bus adapter	Fibre Channel host bus adapter
Network interface controller (NIC) and iSCSI software initiator	Fibre Channel host bus adapter
IP switch	Fibre Channel switch
IP router
iSCSI name, such as IQN (iSCSI qualified name) or EUI (extended-unique identifier)	WWNN (worldwide node name)

iSCSI initiators and targets

In an iSCSI configuration, the iSCSI host or server sends requests to a node. The host contains one or more initiators that attach to an IP network to initiate requests to and receive responses from an iSCSI target. Each initiator and target are given a unique iSCSI name such as an iSCSI qualified name (IQN) or an extended-unique identifier (EUI). An IQN is a 223-byte ASCII name. An EUI is a 64-bit identifier. An iSCSI name represents a worldwide unique naming scheme. This scheme identifies each initiator or target in the same way that worldwide node names (WWNNs) are used to identify devices in a Fibre Channel fabric.

iSCSI targets are devices that respond to iSCSI commands. An iSCSI device can be an end node such as a storage device, or it can be an intermediate device such as a bridge between IP and Fibre Channel devices. Each iSCSI target is identified by a unique iSCSI name. The Lenovo Storage V7000 can be configured as one or more iSCSI targets. Each node canister that has one or both of its node Ethernet ports that are configured becomes an iSCSI target.

To transport SCSI commands over the IP network, an iSCSI driver must be installed on the iSCSI host and target. The driver is used to send iSCSI commands and responses through a network interface controller (NIC) or an iSCSI HBA in the host or target hardware.

For maximum performance, use a gigabit Ethernet adapter that transmits 1000 megabits per second (Mbps) for the connection between the iSCSI host and the iSCSI target.

iSCSI host connection options

Figure 1 shows an iSCSI host that connects to Lenovo Storage V7000 over an Ethernet network.

Figure 1. Transmitting SCSI over TCP/IP
iSCSI local SAN

Figure 2 shows an example where the iSCSI host still connects to an Ethernet network, but a bridge or gateway continues the connection on the Fibre Channel network. The bridge or gateway serves to translate between the Ethernet and Fibre Channel connections so that the iSCSI host detects the Lenovo Storage V7000 system as an iSCSI target.

Figure 2. Transmitting SCSI over both TCP/IP and Fibre Channel interconnections
This figure depicts an iSCSI heterogeneous IP SAN

iSCSI multisession support

Multisession support, or host-level multipath support, provides multiple paths between iSCSI initiators and targets, which is useful for high availability and load balancing.

Some products require that an iSCSI initiator login with a specific IP address of the target versus logging in with a target name. Logging in with a target name in those products causes the iSCSI initiator to log in to all of the IP addresses in the target and causes session reinstantiation that results in session loss of the previous login.

Lenovo Storage V7000 offers single subnet multi-session configurations and multiple and dual subnet multisession configurations that remove the restriction against logging in to a target name. iSCSI initiators use Internet storage name service (iSNS) servers to log in, which enables the discovery of the iSCSI targets and their IP addresses.

An iSCSI session is a TCP relationship between an iSCSI initiator node port and an iSCSI target node port. When established, iSCSI control, data, and status messages are communicated over the session.
Each session is identified with multiple iSCSI initiator names and the target name plus the iSCSI initiator session ID (ISID) plus the target portal group tag (target-side ID).
The Lenovo Storage V7000 multi-session approach is to have relationships between multiple initiator instances (separate ISIDs) to a single target portal group in an iSCSI target. This approach is similar to having multiple SCSI initiator ports from the same iSCSI initiator device have relationships with a single SCSI target port.