Remote Access Devices

Internet SCSI (iSCSI) is a network protocol standard, officially ratified on 2003-02-11 by the Internet Engineering Task Force, that allows the use of the SCSI protocol over TCP/IP networks. iSCSI is a transport layer protocol in the SCSI-3 specifications framework. more...

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Other protocols in the transport layer include SCSI Parallel Interface and Fibre Channel.

Acceptance of iSCSI in corporate production environments has accelerated now that Gigabit Ethernet is common. Building iSCSI-based Storage Area Networks (SAN) has become a less costly but worthy alternative to creating Fibre Channel-based SANs.

Functionality

The iSCSI (pronounced eye-skuzzy) protocol uses TCP/IP for its data transfer. Unlike other network storage protocols, such as Fibre Channel (which is the foundation of most SANs), it requires only the simple and ubiquitous Ethernet interface (or any other TCP/IP-capable network) to operate. This enables low-cost centralization of storage without all of the usual expense and incompatibility normally associated with Fibre Channel storage area networks.

Critics of iSCSI expect worse performance than Fibre Channel due to the overhead added by the TCP/IP protocol to the communication between client and storage. However new techniques like TCP Offload Engine (TOE) help in reducing this overhead. Tests have shown excellent performance of iSCSI SANs, whether TOEs or plain Gigabit Ethernet NICs were used. In fact, in modern high-performance servers, a plain NIC with efficient network driver code can outperform a TOE card because fewer interrupts and DMA memory transfers are required. Initial iSCSI solutions are based on a software stack. The iSCSI market is growing steadily, and should improve in performance and usability as more organizations deploy Gigabit and 10 Gigabit networks, and manufacturers integrate iSCSI support into their operating systems, SAN products and storage subsystems. iSCSI becomes even more interesting since Ethernet is starting to support higher speeds than Fibre Channel.

Storage Devices

In the context of computer storage, iSCSI allows a machine to use an iSCSI initiator to connect to remote targets such as disks and tape drives on an IP network for block level I/O. From the point of view of the class drivers and application software, the devices appear as locally attached SCSI devices. More complex environments consisting of multiple hosts and/or devices are called Storage Area Networks (SAN).

iSCSI devices should not be confused with Network-Attached Storage (NAS) devices which include server software to handle the arbitration of simultaneous access requests from different hosts. Allowing multiple hosts to have simultaneous access to a single device is a difficult task common to all SCSI devices. Without host-to-host communication, each host is unaware of the state and intentions of the other hosts. This condition leads to data corruption and race conditions. In the realm of disk storage, cluster software solves this issue.

Read more at Wikipedia.org