Chapter 1. Disk attachment technology 9
򐂰 Ethernet transfer rate is growing. 10 Gbps Ethernet is coming and it gains wider
commercial acceptance. Migrating to 10 GbE can significantly increase the performance
of an iSCSI infrastructure.
1.3.1 iSCSI initiators and targets
iSCSI uses the concept of initiators and targets, as shown in Figure 1-6.
Figure 1-6 iSCSI components
The protocol allows clients (called initiators) to send SCSI commands (CDBs) to SCSI
storage devices (targets) on remote servers.
An initiator functions as an iSCSI client. An initiator typically serves the same purpose to a
computer as a SCSI bus adapter would, except that instead of physically cabling SCSI
devices (like hard drives and tape changers), an iSCSI initiator sends SCSI commands over
an IP network. An initiator falls into two broad types:
򐂰 Software initiator
A software initiator uses code to implement iSCSI. Typically, this happens in a
kernel-resident device driver that uses the existing network card (NIC) and network stack
to emulate SCSI devices by utilizing the iSCSI protocol. Software initiators are available for
most operating systems, and are the most common method of deploying iSCSI.
An example of an iSCSI stack is the Microsoft iSCSI Software Initiator, which runs on
Windows Server 2003, Windows Server 2008, and VMware ESX4. At the time of writing,
the current version is V2.08 and is available for download from the Microsoft website. For
Windows Sever 2008 and ESX4, the iSCSI initiator is included in the box.
Note: Refer to System Storage Operation Center (SSIC) for the complete list of the
supported operating systems. The SSIC can be found at:
10 IBM System Storage DS3500 Introduction and Implementation Guide
For the IBM AIX operating system, refer to the “iSCSI software initiator and software
target” topic at the following URL:
򐂰 Hardware initiator
A hardware initiator uses dedicated hardware, typically in combination with software
(firmware) running on that hardware, to implement iSCSI. A hardware initiator mitigates
the overhead of iSCSI and TCP processing and Ethernet interrupts, and therefore might
improve the performance of servers that use iSCSI.
An iSCSI host bus adapter (more commonly, HBA) implements a hardware initiator. A
typical HBA is packaged as a combination of a Gigabit (or 10 Gigabit) Ethernet NIC, a
TCP/IP offload engine (TOE) technology, and a SCSI bus adapter, which is how it appears
to the operating system. Inside the operating system, the iSCSI HBAs are classified as
storage adapters.
An iSCSI HBA can include PCI option ROM to allow booting from an iSCSI target.
A TCP Offload Engine, or “TOE Card”, offers an alternative to a full iSCSI HBA. A TOE
“offloads” the TCP/IP operations for this particular network interface from the host
processor, freeing up CPU cycles for the main host applications. When a TOE is used
rather than an HBA, the host processor still has to perform the processing of the iSCSI
protocol layer itself, but the CPU overhead for that task is low.
iSCSI HBAs or TOEs are used when the additional performance enhancement justifies the
additional expense of using an HBA for iSCSI, rather than using a software-based iSCSI
client (initiator).
An iSCSI target usually represents hard disk storage that works over the IP or Ethernet
networks. DS3500. Other types of peripheral devices, like tape drives and medium changers,
can act as iSCSI targets as well.
iSCSI naming
The iSCSI initiators and targets on a SAN are known by their respective iSCSI names, which
must be unique. The iSCSI name is used as part of an ISCSI address, and as part of all
sessions established between initiators and targets. The types of iSCSI names are:
A commonly used format of iSCSI names is the iSCSI Qualified Name (IQN). The format of
an IQN is:
iqn.yyyy-mm.{reversed domain name}
For example, an iSCSI HBA inside a host server named Rhine in the domain rivers.local
would be assigned the following IQN:
The DS3500 uses IQN names.
An alternative type of iSCSI name is the Enterprise Unique Identifier (EUI). The format of an
EUI is
eui plus 16 hex digits. For example:

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