Editing ISCSI (section)

==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.

==={{Anchor|ISOE-CARD}}Initiator===
{{Further|SCSI 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 (operating system)|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 [[Transmission Control Protocol|TCP]] processing and [[interrupt|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 [[Conventional PCI|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,<ref>{{cite web|url=http://www.chelsio.com/chelsio-demonstrates-next-generation-40g-iscsi-at-snw-spring/ |title=Chelsio Demonstrates Next Generation 40G iSCSI at SNW Spring |publisher=chelsio.com |date=2013-04-03 |access-date=2014-06-28}}</ref> 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===
{{See also|SCSI 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|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.<ref>[http://roc.cs.berkeley.edu/papers/inet-computing.pdf Architecture and Dependability of Large-Scale Internet Services] David Oppenheimer and David A. Patterson, Berkeley, ''IEEE Internet Computing'', September–October 2002.</ref>

====Software target====
{{See also|List of storage area network management systems}}

Nearly all modern mainstream server operating systems (such as [[BSD]], [[Linux]], [[Solaris (operating system)|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====
{{Main|Logical unit number}}
In [[SCSI]] terminology, LU stands for ''logical unit'', which is specified by a unique ''[[logical unit number]]''. A LUN represents an individually addressable (logical) SCSI device that is part of a physical SCSI device (target). In an iSCSI environment, LUNs are essentially numbered disk drives. An initiator negotiates with a target to establish connectivity to a LUN; the result is an iSCSI connection that emulates a connection to a SCSI hard disk. Initiators treat iSCSI LUNs the same way as they would a raw SCSI or IDE hard drive; for instance, rather than mounting remote directories as would be done in [[Network File System (protocol)|NFS]] or [[CIFS]] environments, iSCSI systems format and directly manage filesystems on iSCSI LUNs.

In enterprise deployments, LUNs usually represent subsets of large [[RAID]] disk arrays, often allocated one per client. iSCSI imposes no rules or restrictions on multiple computers sharing individual LUNs; it leaves shared access to a single underlying filesystem as a task for the operating system.

===Network booting===
For general data storage on an already-booted computer, any type of generic network interface may be used to access iSCSI devices.{{Citation needed|date=November 2013}} However, a generic consumer-grade network interface is not able to boot a [[diskless]] computer from a remote iSCSI data source.{{Citation needed|date=November 2013}} Instead, it is commonplace for a server to load its initial operating system from a [[TFTP]] server or local boot device, and then use iSCSI for data storage once booting from the local device has finished.{{Citation needed|date=November 2013}}

A separate [[DHCP server]] may be configured to assist interfaces equipped with [[network boot]] capability to be able to boot over iSCSI. In this case, the network interface looks for a DHCP server offering a [[Preboot Execution Environment|PXE]] or [[bootp]] boot image.<ref>{{cite web| url=http://ipxe.org/howto/chainloading| title=Chainloading iPXE| access-date=2013-11-11| publisher=ipxe.org}}</ref> This is used to kick off the iSCSI remote boot process, using the booting network interface's [[MAC address]] to direct the computer to the correct iSCSI boot target{{Citation needed|date=November 2013}}. One can then use a software-only approach to load a small boot program which can in turn mount a remote iSCSI target as if it was a local SCSI drive and then fire the boot process from said iSCSI target{{Citation needed|date=November 2013}}. This can be achieved using an existing [[Preboot Execution Environment]] (PXE) boot ROM, which is available on many wired Ethernet adapters. The boot code can also be loaded from CD/DVD, floppy disk (or floppy disk image) and USB storage, or it can replace existing PXE boot code on adapters that can be re-flashed.<ref>{{cite web| url=http://ipxe.org/howto/romburning| title=Burning iPXE into ROM| access-date=2013-11-11| publisher=ipxe.org}}</ref> The most popular free software to offer iSCSI boot support is [[iPXE]].<ref>{{cite web| url=http://ipxe.org/start| title=iPXE - Open Source Boot Firmware| access-date=2013-11-11| publisher=ipxe.org}}</ref>

Most Intel Ethernet controllers for servers support iSCSI boot.<ref>{{cite web|url=http://www.intel.com/content/www/us/en/ethernet-controllers/ethernet-controllers.html |title=Intel Ethernet Controllers |publisher=Intel.com |access-date=2012-09-18}}</ref>

===Addressing===
iSCSI uses TCP (typically [[TCP and UDP port|TCP ports]] 860 and 3260) for the protocols itself, with higher-level names used to address the objects within the protocol.
Special names refer to both iSCSI initiators and targets. iSCSI provides three name-formats:

; iSCSI Qualified Name (IQN)
: Format: The iSCSI Qualified Name is documented in RFC 3720, with further examples of names in RFC 3721. Briefly, the fields are:

:* literal iqn (iSCSI Qualified Name)
:* date (yyyy-mm) that the naming authority took ownership of the domain
:* reversed domain name of the authority (e.g. org.alpinelinux, com.example, to.yp.cr)
:* Optional ":" prefixing a storage target name specified by the naming authority.

:From the RFC:{{Ref RFC|3720|section=3.2.6.3.1|p=32|quote=Type "iqn." (iSCSI Qualified Name)}}
{|class="wikitable"
! Type !! . !! Date !! . !! Naming Auth !! : !! String defined by ''example.com'' Naming Authority
|-
| iqn || . || 1992-01 || . || com.example || : || storage:diskarrays-sn-a8675309
|-
| iqn || . || 1992-01 || . || com.example
|-
| iqn || . || 1992-01 || . || com.example || : || storage.tape1.sys1.xyz
|-
| iqn || . || 1992-01 || . || com.example || : || storage.disk2.sys1.xyz
|}

; Extended Unique Identifier (EUI)
: Format: eui.{EUI-64 bit address} (e.g. <code>eui.02004567A425678D</code>)
; T11 Network Address Authority (NAA)
: Format: naa.{NAA 64 or 128 bit identifier} (e.g. <code>naa.52004567BA64678D</code>)

IQN format addresses occur most commonly. They are qualified by a date (yyyy-mm) because domain names can expire or be acquired by another entity.

The IEEE Registration authority provides EUI in accordance with the EUI-64 standard. NAA is part OUI which is provided by the IEEE Registration Authority. NAA name formats were added to iSCSI in RFC 3980, to provide compatibility with naming conventions used in [[Fibre Channel]] and [[Serial Attached SCSI]] (SAS) storage technologies.

Usually, an iSCSI participant can be defined by three or four fields: 
# Hostname or IP Address (e.g., "iscsi.example.com")
# Port Number (e.g., 3260)
# iSCSI Name (e.g., the IQN "iqn.2003-01.com.ibm:00.fcd0ab21.shark128")
# An optional [[Challenge-handshake authentication protocol|CHAP]] Secret (e.g., "secretsarefun")

===iSNS===
{{Main|Internet Storage Name Service}}
iSCSI initiators can locate appropriate storage resources using the [[Internet Storage Name Service]] (iSNS) protocol. In theory, iSNS provides iSCSI SANs with the same management model as dedicated [[Fibre Channel]] SANs. In practice, administrators can satisfy many deployment goals for iSCSI without using iSNS.