iSCSI is Internet SCSI (Small Computer System Interface). We can mount block devices (disks) across an IP network to local system and then use them like any other block device. iSCSI is a client-server protocol.
The server-side is referred to as the ‘target,’ while the client-side is referred to as the ‘initiator.’ Both the target and initiator are uniquely identified by a string called the iSCSI Qualified Name (iQN).
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In computing, iSCSI is an acronym for Internet Small Computer System Interface, an Internet protocol(IP)-based storage networking standard for linking data storage facilities.
By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. iSCSI can be used to transmit data over Local Area Network(LANs), Wide Area Network(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 SCSI storage devices (targets) on remote servers. It is a storage area network(SAN) protocol, allowing organizations to consolidate storage into data center storage arrays while providing hosts (such as database and web servers) with the illusion of locally attached disks. Unlike traditional Fibre Channel, which requires special-purpose cabling, iSCSI can be run over long distances using existing network infrastructure. iSCSI was pioneered by IBM and Cisco in 1998 and submitted as draft standard in March 2000.
iSCSI allows two hosts to negotiate and then exchange SCSI commands using IP networks.
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A storage area network(SAN) is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to enhance storage devices, such as disk array, tape libraries, and optical jukebox, accessible to servers so that the devices appear like locally attached device to the operating system.
Network Attached Storage (NAS) was designed before the emergence of SAN as a solution to the limitations of the traditionally used Direct Attached Storage (DAS), in which individual storage devices such as disk drives are connected directly to each individual computer and not shared.
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.
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.