What is iSCSI (Internet Small Computer System Interface)?

Internet Small Computer System Interface is a transport layer protocol built on TCP Protocol. It ensures data transfer between the iSCSI initiator and the storage target over the TCP/IP network at the block level. It also supports high-level encryption of the transferred data packets and decryption on the arrival of data packets at the target side.

What is iSCSI (Internet Small Computer System Interface)?

The Internet Small Computer System Interface, or iSCSI, is an IP-based protocol for transmitting data and linking data storage facilities. It works for transmitting data over several types of networks, including LAN (local area networks), WAN (wide area networks), and the Internet itself. It is also a SAN (Storage Area Network) protocol, which can allow companies to utilize data center storage arrays to consolidate their data. Doing so can make it look as if they are utilizing local hard disk storage versus a data center storage, due to the improved performance in data retrieval that a SAN provides.

The first iSCSI storage device was introduced for use with the Windows NT, 2000, and Linux operating systems in 2001 by IBM. Since then, iSCSI devices have been made available for other operating systems, including Solaris, NetWare, VMware, macOS, NetBSD, and HP-UX. Most iSCSI devices are hard disk-based, but there are also some iSCSI tape and medium changer devices on the market as well.

How does it work?

ISCSI works by transporting block-level data between an iSCSI initiator on a server and an iSCSI target on a storage device. The iSCSI protocol encapsulates SCSI commands and assembles the data in packets for the TCP/IP layer. Packets are sent over the network using a point-to-point connection. Upon arrival, the iSCSI protocol disassembles the packets, separating the SCSI commands so the operating system (OS) will see the storage as if it was a locally connected SCSI device that can be formatted as usual. Today, some of iSCSI’s popularity in small to midsize businesses (SMBs) has to do with the way server virtualization makes use of storage pools. In a virtualized environment, the storage pool is accessible to all the hosts within the cluster and the cluster nodes communicate with the storage pool over the network through the use of the iSCSI protocol. There are a number of iSCSI devices that enable this type of communication between client servers and storage systems.

iSCSI Components

iSCSI initiator

The initiator is the host-based software or hardware installed in the server and enables the sending of data to and from the storage array. The source array also can act as an initiator for data migration between the storage arrays. Standard Ethernet components can be used to create the storage network for the software initiator, and iSCSI initiators manage multiple and parallel communication links to multiple targets.

iSCSI target

The target is the system placed on a storage device, essentially a server for hosting the storage resources and allowing access to the storage. The iSCSI targets are the storage resources located on an iSCSI server, which usually represents hard disk storage and is often accessed by Ethernet-based networks. Targets are data providers such as disk arrays or tape libraries. iSCSI targets expose one or more SCSI LUNs to specific iSCSI initiators, but for enterprise storage, iSCSI targets are logical entities. iSCSI targets manage multiple and parallel communication links to multiple initiators.

An iSCSI host bus adapter (HBA) is similar to a Fibre Channel, which offloads more processing from the system processor. HBA helps improve the performance of the storage and server network, but its cost is typically three or four times higher than a standard Ethernet NIC. However, the iSCSI offload engine (iSOE) is similar and less expensive, which can be a good alternative.

Benefits of using iSCSI

With the combination of SCSI, Ethernet, and TCP/IP, iSCSI offers the following benefits:

  • As iSCSI is built on stable and familiar standards, most of the IT staff are familiar with the technology.
  • It creates a SAN, which in turn reduces the Total Cost of Ownership (TCO). As SAN systems are easy to install and require less maintenance due to the involvement of TCP/IP, the need for hiring specialized personnel is eliminated; consequently, the costs are considerably reduced.
  • As Ethernet transmissions can travel over the IP network, there will be no practical distance limitations.
  • Its deployment offers a high degree of interoperability. So, it reduces disparate networks and cabling as it can be achieved through Ethernet switches instead of the specialized hardware of Fibre Channel (FC) switches which are high on price factor as well.
  • Unlike fiber channels, it doesn’t operate over a separate network protocol. It transmits data using the TCP/IP protocol over familiar Ethernet networks that are supporting network traffic in data centers right now.
  • Little or no investment in new network technology is needed to start using an iSCSI SAN. However, some deployments of dedicated iSCSI networks or subnets maximize the bandwidth available for storage. It is also common for an iSCSI network to be deployed on fast Ethernet such as 10 Gigabit (or more) optimized performance and IOPS.
  • The iSCSI protocol supports many features to improve security and performance such as immutable snapshots, deduplication, thin provisioning, etc.

iSCSI limitations

The main limitation of iSCSI storage networks is related to their performance compared to FC-based storage environments. When it was first rolled out, the performance gap between the two technologies was profound, but the availability of 10 GbE iSCSI and other tech implementations such as multipathing and data center bridging has helped to close the performance gap.

Today, iSCSI storage performs as well as — or nearly as well as — similarly configured FC systems. And with 100 GbE connectivity for iSCSI on the near horizon, iSCSI storage will likely keep pace with — or possibly outrun — FC systems running at 32 GBps and 64 GBps.


To conclude, iSCSI storage can leverage an enterprise’s network infrastructure investments by using standard Ethernet switches and routers to move the data from server to storage. It also enables IP infrastructure to be used for expanding access to SAN storage and extending SAN connectivity across any distance. So organizations requiring data storage security or disaster recovery will benefit a lot from iSCSI SAN.


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