What Is iSCSI? Definition, Components, & Performance

Internet Small Computer Systems Interface or iSCSI is a storage area networking (SAN) protocol. It is an Internet Protocol-based networking standard for transferring data carrying SCSI commands over a TCP/IP network. iSCSI links data storage facilities and provides block-level access to storage devices. iSCSI was pioneered by IBM and Cisco as a proof of concept in 1998, a draft standard submitted to the Internet Engineering Task Force (IETF) in 2000, and ratified in 2003.

How does iSCSI work?

iSCSI allows two hosts to interpose and exchange SCSI commands by using IP networks that take a high-performance local storage bus, emulate it over a network, and create a storage area network (SAN). The protocol encapsulates SCSI commands, assembles data in TCP/IP layer packets, and sends them using a point-to-point connection.

iSCSI performs by transporting block-level data between an iSCSI initiator and an iSCSI target, the iSCSI initiator placed on a server, and the iSCSI target placed on a storage device. After the packet arrives at the iSCSI target, the protocol disassembles the packets and separates the SCSI commands so the storage can be visible to the operating system (OS).

iSCSI can run over existing IP infrastructure and does not require any dedicated cabling; as a result, iSCSI is often seen as low-cost than its alternative, such as Fibre Channel. The iSCSI protocol can communicate with arbitrary types of SCSI devices, and system administrators widely use the protocol to allow servers to access disk volumes on storage arrays. But the performance can be degraded if iSCSI is not operated on a dedicated network or subnet.

Read more: What is SAN?

iSCSI Components

iSCSI SAN consists of two basic components: the iSCSI initiators and iSCSI targets nodes. Other components are also used, such as Host-based Adopters (HBA) and iSCSI offload engine (iSOE).

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. 

iSCSI Performance

For fast and massive data transfer, iSCSI performance makes it widely acceptable for enterprises. In a shared-storage network with multiple servers and clients, iSCSI allows access to central storage resources as a locally connected device. Some important factors may affect iSCSI performance, such as TCP Delayed ACK, iSCSI session login balance, ethernet jumbo frame, network bottleneck or oversubscription, PFC (Priority flow control) setting, and bandwidth allocation on the network. 

Data transfer Speed

iSCSI implementations should be considered 10 gigabits per second Ethernet (GbE) infrastructure as the base configuration for performance, but many iSCSI implementations are based on a 1 GbE infrastructure. iSCSI speed performance depends on network speed, storage ethernet controller speed, controller configuration, controller port count, physical drives and RAID level inside the SAN, number of ports used on host servers, physical network configurations, and other factors.

Massive data transfer

The massive data transfer performance makes iSCSI an effective alternative to Fibre Channel for handling Ethernet storage. iSCSI protocol can be run over with a lossless form of ethernet that provides a reliable connection, which helps to improve the performance of iSCSI. You should ensure proper bandwidth is dedicated to iSCSI on the network, which is important for good performance.

Multipathing Transport

In multipathing, the technique host can have more than one physical path to transferring data between the host server and storage. iSCSI path from a host consists of an iSCSI adapter, switch ports, connecting cables, and a storage controller port. In multipathing, the host can select another available path when any of the components fail. Multipathing increases input/output speeds at iSCSI network packets and accelerates data transmission.

Jumbo Frames

The Ethernet Jumbo frames protocol allows iSCSI storage systems to transfer larger amounts of data. Shipping bigger chunks of data simultaneously improves performance. The 9000-byte frames relieve congestion on slower Ethernet networks and help performance boost by about 10-20 percent. Enabling jumbo frames and disabling the TCP delayed acknowledgment feature for iSCSI may increase iSCSI performance instantly.

iSCSI vs. Fiber Channel: Storage Data Transmission Methods

iSCSI and Fiber Channel are both effective and popular for transferring a large amount of data. Some major differences between iSCSI and Fiber Channel are given below. 

  iSCSI Fiber Channel
Infrastructure The same infrastructure is deployed in the user and server network Dedicated infrastructure separated user and server network
CPU Load Higher than Fiber Channel Lower than iSCSI
Supported Distance Distance is not limited Limited distance
Deployment Easy to manage and simple Labor-intensive and complex
Latency Higher than Fiber Channel Lower than iSCSI
Cost Less Expensive More Expensive
Applicability Well-suitable for low input/output application Well-suitable for high input/output application

When to Implement iSCSI Over Fibre Channel

Fibre Channel SAN is great for businesses with enough budget and expertise to implement and manage a dedicated fiber channel network. iSCSI SANs run on Ethernet networks and do not always need dedicated network hardware. iSCSI SANs are good for businesses that require high-performance workloads without deploying dedicated network hardware.

5 Benefits of iSCSI

As a transport layer protocol, iSCSI is built on top of TCP Protocol, ensures data transfer over the TCP/IP network at the block level, and supports high-level data packet encryption. Some key benefits of the iSCSI protocol are given below. 


iSCSI is cost-effective and provides a cheap network at the block level compared to Fibre Channel. The system helps to reduce additional network devices. iSCSI does not always need to use HBAs, separate cabling, or other specific storage network devices.

Flexible and reusable

It runs on an internet protocol that makes it more flexible and does not limit the distance between the initiator and to target. iSCSI leverages the interoperability advantages of TCP/IP and Ethernet. It is reusable, and existing servers can be used multiple times to configure iSCSI implementation.

Fast and Large data Transfer

iSCSI is faster and more efficient; it can be used for massive data transmission systems. iSCSI is usually configured for 10 gigabits per second Ethernet (GbE) infrastructure and can transfer a massive amount of data. It can be run over with a lossless form of Ethernet.

Easy Deployment and Manage

iSCSI is easy to understand and configure, and users don’t need much knowledge to maintain the system. It is well-suited to support development and disaster recovery. iSCSI components have just a few interoperability challenges compared to Fiber Channel.


The iSCSI protocol ensures network security by enabling authentication identity, logical and physical network isolation, integrity, and confidentiality. The network isolation ensures valid initiators connect to storage arrays, and integrity and confidentiality protect data from unauthorized access.

What are iSCSI Limitations?

iSCSI protocol helps improve performance; however, it has some limitations. Some of the limitations of iSCSI are:  

Imports only normal LUNs

The Isilon iSCSI module does not support importing snapshot LUNs and clone LUNs; it only supports importing normal LUNs. The user cannot restore a snapshot or clone LUN and does not support replicating to another cluster.

Requires System Reboot

iSCSI VLANs do not allow editing, adding, or removing for an iSCSI-booted device. The NCU needs a system reboot for L2ND or device configurations of the iSCSI-booted adapters, and even any changes to a teaming configuration of the iSCSI-booted device need a system reboot.

Limited Component Support

Most HP NC-Series 10 GbE adapters do not support iSCSI, although iSCSI is supported on FlexNIC-enabled adapters. An iSCSI HBA helps improve the performance, but it is expensive, typically three or four times higher than a standard Ethernet NIC.

iSCSI and Storage Targets

Though iSCSI is very effective for data transmission, many physical devices have not yet featured native iSCSI interfaces on the component level. Some manufacturers use iSCSI RAIDs as a target. Most iSCSI targets involve disks, and the iSCSI tape and medium-changer targets are also popular. 

Storage Area Networks

Storage Area Network (SAN) is an independent dedicated network that provides access to consolidated data storage in block-level format. The host servers use iSCSI to transport block-level data to the SANs, primarily used to make storage devices accessible to servers. The storage devices appear as locally attached devices to the operating systems. The recommended hardware for implementing an iSCSI SAN is TCP/IP connectivity between the SAN storage pool and the Servers. Implementing a dedicated, high-speed network ensures the best performance, which can be physical or virtual by design. iSCSI SAN connectivity needs specific software to utilize resources from the shared storage pool.

Network Attached Storage

Network Attached Storage (NAS) supports iSCSI targets. Nas is generally provided by a specific appliance running a specific operating system. The appliances usually contain one or more hard disk drives and are often divided into logical, redundant storage arrays. The NAS Appliance is the dedicated storage server that attaches directly to a common network by an ethernet connection with an assigned IP Address. Several appliances can be added to the LAN as needed, but more can create additional strain on oversubscribed ethernet LANs. NAS Appliances can provide massive data storage compared to their SAN counterparts, and their high-end performance is becoming common at the enterprise level.

Read more: What is NAS?

LUN Storage Target

For iSCSI, the storage systems are targets and have storage target devices in SAN environments; the devices are referred to as logical units. A logical unit number (LUN) identifies a logical unit addressed by the SCSI protocol or SAN protocols encapsulating SCSI. Users can configure the storage by creating LUNs for iSCSI, which are then accessed by hosts using iSCSI protocols. To connect iSCSI networks, the hosts can use dedicated iSCSI HBAs, standard Ethernet network adapters, converged network adapters (CNAs), or TCP offloads engine cards with software initiators. LUNs can be used with devices that support read/write operations, such as tape drives.

Virtual Storage Targets

The virtual tape library (VTL) uses disk storage for storing data written to virtual tapes, and the virtual targets can be presented using iSCSI target software, internal controllers, or external bridges. Many physical devices are generally not featured for native iSCSI interfaces on the component level, and Virtualizing disk and tape targets can be easier than representing an actual physical device.

Bottom Line: Internet Small Computer Systems Interface

Data transfer has become an important part in the last few decades; nowadays, massive data transfer is becoming necessary. Choosing the right SAN for business may focus on performance requirements, management expertise, availability of resources, and budget. Since iSCSI SANs run on Ethernet networks, this makes a perfect choice for businesses requiring high-performance workloads without spending dedicated network hardware.

Read more: What Is a Hard Disk Drive (HDD) & How Does It Work?

Al Mahmud Al Mamun
Al Mahmud Al Mamun
Al Mahmud Al Mamun is a writer for TechnologyAdvice. He earned his B.S. in computer science and engineering from Prime University, Bangladesh. He attained more than 25 diploma courses and 100 certificate courses. His expertise and research interests include artificial intelligence (AI), artificial neural networks, and convolutional neural networks.
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