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.

Connections from iSCSI-attached hosts to system nodes are supported. iSCSI connections from system nodes to IBM® Storage FlashSystem, IBM Spectrum Accelerate, and Dell EqualLogic systems that are used as external storage systems are also supported. If the external storage is not in this list, then raise SCORE approval request.

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.

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

iSCSI host connection options

Figure 1 shows an iSCSI host that connects to the system over an Ethernet network.
Figure 1. Transmitting SCSI over TCP/IP
Transmitting SCSI over TCP/IP

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 hosts 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 hosts 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.

The system 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 system 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.

iSCSI storage support

The system supports iSCSI connections to IBM Storage FlashSystem and Dell EqualLogic systems that are used as external storage systems. For specific configuration information, see iSCSI configuration details for host connections.

Discovery and path configuration for iSCSI controllers is done as follows:

  • Specify only a target IP and issue a discovery request to the iSCSI target controller
  • The target controller returns a list of all configured iSCSI qualified names (IQNs) on that controller
  • Establish connections to the discovered target IQNs