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12 IBM Midrange System Storage Hardware Guide

The logical link that carries the commands and data to and from TCP/IP endpoints is called

iSCSI session. A session is made up of at least one TCP/IP connection from an initiator

(host)

to a target (storage device), as shown in Figure 1-5. The iSCSI initiator can be either an

iSCSI HBA inside a host server, or you can define a software iSCSI initiator by using an iSCSI

stack and an Ethernet network adapter. An example of an iSCSI stack is the Microsoft iSCSI

Software Initiator, which runs on Windows Server 2003 and Windows Server 2008.

Figure 1-5 iSCSI components

iSCSI SANs might not be a solution for everyone, given that iSCSI SANs might not deliver the

same performance that Fibre Channel SANs can deliver. However, there are ways to address

iSCSI performance:

򐂰 While the host servers can use almost any Ethernet network interface card for iSCSI

traffic, this does mean that the CPUs on the host server have to run the iSCSI stack (to

perform encapsulation of SCSI commands and data). This causes increased CPU and

memory processing, which can impact performance. For increased performance, it is

better to use dedicated iSCSI HBAs to process the TCP/IP stack. This technology is

known as TCP Offload Engine (TOE). TOE technology relieves the CPUs on the host

server from having to process the SCSI encapsulation, which can lead to better

performance.

򐂰 To avoid excessive traffic, but mainly to ensure that only valid initiators connect to storage

servers, it is better to run iSCSI over dedicated network segments or virtual LANs (VLAN).

1.7 Fibre Channel direct/switch basics

Fibre Channel (FC) is a high-speed disk attachment technology, designed to connect a large

number of storage devices to a number of host servers across a Storage Area Network

(SAN). Fibre Channel Protocol (FCP) transfers SCSI commands and data across physical FC

links.

Target

Initiator

Application

Server

Application

Server

DS5000

Storage Server

SCSI (Block I/O) Protocol

iSCSI Device

Discovery

IBM Midrange System DS4000 Series User Manual

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