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IP Storage Networking – An Overview

IP Storage Networking refers to computer systems and storage elements that are connected via IP. It also refers to the IP infrastructure transporting storage traffic among these targets.

Many computer systems and storage assets have native IP interfaces. These could include servers, RAID or tape libraries with an iSCSI adapter or iSCSI controller. These interfaces typically are Ethernet and include protocol processing, such as TCP/IP offload engines, to reduce processing loads on the host devices.

iSCSI Connection Devices

There a number of technology implementations offering options for connecting iSCSI. These include traditional NICs (Network Interface Cards), NICs with TOEs (TCP Offload Engines) and iSCSI HBAs (Host Bus Adapters).

iSCSI – Remote Connectivity

iSCSI can be deployed for linking SANs over MAN (Metropolitan Area Network) and WAN (Wide Area Network) environments using TCP/IP as the delivery mechanism. This is not limited to extending native iSCSI SANs but can also be used to Fibre Channel SANs leveraging existing wide area technologies as extending SANs over long distances using iSCSI requires only standard Ethernet equipment. When connecting to Fibre Channel SANs, a bridging device will be needed. Such device can be stand alone or incorporated into some Fibre Channel switches. Such functionality in Fibre Channel switches facilitates Fibre Channel to Fibre Channel switching, Fibre Channel to iSCSI IP switches and router have no equivalent functionality.

iSCSI Operation

iSCSI is based on the SCSI architecture. In the SCSI-3 implementation of the standard an initiator (typically a host system such as a server) issues requests to read or write data. A data storage resource such as a RAID sub-system responds to that request. The storage device is a target and typically responds to initiator requests to read or write. A target has one or more logical units (identified by Logical Unit Numbers – LUNs) that respond to commands from the initiator.

A command sent by an initiator (server) to a specific logical unit (target) could be a command to read a specified number of data blocks. The transfer of data to or from target’s logical unit takes place and is terminated by a status to indicate completion of the request.

The challenge for iSCSI is to encapsulate and reliably deliver SCSI commands and data between initiators and targets over TCP/IP networks. The iSCSI protocol initiative must deal with the inherent contradiction posed by the requirements of SCSI for stable, fast, deterministic communications and the potentially unreliable environment typical of IP networks.

The iSCSI protocol specification details processes for the validation of data and command exchanges and for recovery from data loss that may be inherent in the lower layer transport protocols that underpin TCP/IP transactions. iSCSI monitors the block data transfer and validates completion of the I/O operation. This occurs over one or more TCP connections between initiator and target.

An iSCSI SAN is a combination of native iSCSI initiators such as servers and iSCSI targets such as disk arrays and tape subsystems. Typically each server and storage device supports a Gigabit Ethernet interface and iSCSI protocol stack. This facilitates storage devices being connected directly into Gigabit Ethernet switches and/or IP routers where they appear as any other IP entities in the network. As with normal IP implementations, direct connection to the IP network requires responsibility for device discovery and connection establishment be undertaken by the end device. An iSCSI initiator may have multiple target devices over an IP network, and consequently multiple concurrent TCP connections can be active at the same time.

When using iSCSI, a SCSI initiator must discover storage resources typically through a list of IP addresses of its intended targets. This list can be provided by a lookup table or by a DNS type service such as the Internet Storage Name Service (iSNS). iSNS protocol facilitates device discovery for iSCSI initiators with an iSCSI initiator querying an iSNS server to learn the IP addresses of potential target resources, and then establish TCP/IP connections to them. iSNS servers can be stand alone in the network or distributed across iSCSI devices.

SCSI communication over iSCSI is defined by the SCSI Architectural Model (SAM-2) where iSCSI implements a client-server model between storage targets and initiator servers. Since iSCSI targets and initiators are on an IP network they must conform to the clients/server model and this means having a Network Entity identity which is equivalent to the IP addresses they are assigned. The Network Entity may contain one or more iSCSI devices or servers (iSCSI nodes). An iSCSI Node identifies a SCSI device within a Network Entity that is accessible through the network.