computer science

ISCSI

ISCSI is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand ISCSI rather than just read about it. In short: Internet Small Computer Systems Interface (iSCSI; eye-SKUZ-ee) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP network. iSCSI facilitates data transfers over intranets and to manage storage over long distances. It can be used to transmit data over local area networks (LANs), wid…

Key takeaways

ISCSI belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect ISCSI to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of ISCSI from memory before moving on to harder problems.

Reference excerpt

Internet Small Computer Systems Interface (iSCSI; eye-SKUZ-ee) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP network. iSCSI facilitates data transfers over intranets and to manage storage over long distances. It can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval. The protocol allows clients (called initiators) to send SCSI commands (CDBs) to storage devices (targets) on remote servers. It is a storage area network (SAN) protocol, allowing organizations to consolidate storage into storage arrays while providing clients (such as database and web servers) with the illusion of locally attached SCSI disks. It mainly competes with Fibre Channel, but unlike traditional Fibre Channel which usually requires dedicated cabling, iSCSI can be run over long distances using existing network infrastructure. iSCSI was pioneered by IBM and Cisco in 1998 and submitted as a draft standard in March 2000.

Concepts In essence, iSCSI allows two hosts to negotiate and then exchange SCSI commands using Internet Protocol (IP) networks. By doing this, iSCSI takes a popular high-performance local storage bus and emulates it over a wide range of networks, creating a storage area network (SAN). Unlike some SAN protocols, iSCSI requires no dedicated cabling; it can be run over existing IP infrastructure. As a result, iSCSI is often seen as a low-cost alternative to Fibre Channel, which requires dedicated infrastructure except in its FCoE (Fibre Channel over Ethernet) form. However, the performance of an iSCSI SAN deployment can be severely degraded if not operated on a dedicated network or subnet (LAN or VLAN), due to competition for a fixed amount of bandwidth. Although iSCSI can communicate with arbitrary types of SCSI devices, system administrators almost always use it to allow servers (such as database servers) to access disk volumes on storage arrays. iSCSI SANs often have one of two objectives:

Storage consolidation Organizations move disparate storage resources from servers around their network to central locations, often in data centers; this allows for more efficiency in the allocation of storage, as the storage itself is no longer tied to a particular server. In a SAN environment, a server can be allocated a new disk volume without any changes to hardware or cabling. Disaster recovery Organizations mirror storage resources from one data center to a remote data center, which can serve as a hot / standby in the event of a prolonged outage. In particular, iSCSI SANs allow entire disk arrays to be migrated across a WAN with minimal configuration changes, in effect making storage "routable" in the same manner as network traffic.

Initiator

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: 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 for a computer by speaking the iSCSI protocol. Software initiators are available for most popular operating systems and are the most common method of deploying iSCSI. A hardware initiator uses dedicated hardware, typically in combination with firmware running on that hardware, to implement iSCSI. A hardware initiator mitigates the overhead of iSCSI and TCP processing and Ethernet interrupts, and therefore may 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 network interface controller, some kind of TCP/IP offload engine (TOE) technology and a SCSI bus adapter, which is how it appears to the operating system. An iSCSI HBA can include PCI option ROM to allow booting from an iSCSI SAN. An iSCSI offload engine, or iSOE card, offers an alternative to a full iSCSI HBA. An iSOE "offloads" the iSCSI initiator operations for this particular network interface from the host processor, freeing up CPU cycles for the main host applications. iSCSI HBAs or iSOEs 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). iSOE may be implemented with additional services such as TCP offload engine (TOE) to further reduce host server CPU usage.

Target

The iSCSI specification refers to a storage resource located on an iSCSI server (more generally, one of potentially many instances of iSCSI storage nodes running on that server) as a target. An iSCSI target is often a dedicated network-connected hard disk storage device, but may also be a general-purpose computer, since as with initiators, software to provide an iSCSI target is available for most mainstream operating systems. Common deployment scenarios for an iSCSI target include:

Storage array In a data center or enterprise environment, an iSCSI target often resides in a large storage array. These arrays can be in the form of commodity hardware with free-software-based iSCSI implementations, or as commercial products such as in StorTrends, Pure Storage, HP StorageWorks, EqualLogic, Tegile Systems, Nimble storage, IBM Storwize family, Isilon, NetApp filer, Dell EMC, Kaminario, NS-series, CX4, VNX, VNXe, VMAX, Hitachi Data Systems HNAS, or Pivot3 vSTAC. A storage array usually provides distinct iSCSI targets for numerous clients.

Software target

Nearly all modern mainstream server operating systems (such as BSD, Linux, Solaris or Windows Server) can provide iSCSI target functionality, either as a built-in feature or with supplemental software. Some specific-purpose operating systems implement iSCSI target support.

Logical unit number

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with ISCSI

Start with the simplest possible case. Write down what ISCSI claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to ISCSI before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about ISCSI ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of ISCSI

In research: ISCSI appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses ISCSI in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: ISCSI is common in secondary-school and first-year university syllabi. It links to neighbouring topics Application layer protocols, Ethernet, Network booting, so understanding it makes those chapters shorter.
In everyday life: Look for ISCSI outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study ISCSI in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what ISCSI means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain ISCSI out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is ISCSI in simple terms?

Why does ISCSI matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study ISCSI?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on ISCSI.