22-02-2009, 01:17 AM
INTRODUCTION
The SCSI protocols widely used to access storage devices. The iSCSI protocol is a transport for SCSI over TCP/IP. SM-2 defines an architecture model for SCSI transports, and iSCSI defines such a transport on top of TCP/IP. Other SCSI transports include SCSI Serial and Fibre Channel Protocol (FCP). Until recently standard networking hardware (Ethernet) and IP-based protocols could not provide the necessary high bandwidth and low latency needed for storage access. With the recent advances in Ethernet technology, it is now practical from a performance perspective to access storage devices over an IP network. 1Gb Ethernet is now widely available and is competitive with current 1 and 2 Gb Fibre Channel technology. 10Gb Ethernet will soon also be widely available. Similar to FCP, iSCSI allows storage to be accessed over a storage area network (SAN), allowing shared access to storage. A major advantage of iSCSI over FCP is that iSCSI can run over standard off-the-shelf network components, such as Ethernet. Furthermore, iSCSI can exploit IP-based protocols such as IPSec, for security and Service Location Protocol (SLP) for discovery. A network that incorporates iSCSI SANs need use only a single kind of network infrastructure (Ethernet) for both data and storage traffic, whereas use of FCP requires a separate kind of infrastructure (Fibre Channel) for storage (Fig.2). IP based SANs using iSCSI can be managed using existing and familiar IP-based tools such as Simple Network Management Protocol (SNMP) whereas FCI SANs require specialized management infrastructure. Furthermore, iSCSI-based SANs can extend over arbitrary distances, just like TCP, and are not subject to distance limitations that currently limit FCP.
Fig(1)
Fig.(2) Classic SAN vs iSCSI
In addition to iSCSI, several other protocols have been defined to transport storage over an IP network. FCIP is used to connect separate islands of Fibre Channel SANs over an IP network to form a single unified SAN. iFCP is a gateway-to-gateway protocol for the implementation of Fibre Channel fabric functionality on a network in which TCP/IP switching and routing elements replace Fibre Channel components. Whereas, FCIP and iFCP are used to allow the connection of existing Fibre Channel infrastructures to each other and to IP network. iSCSI enables the creation of SANs complete independent of Fibre Channel.
FACTORS ENABLING IP STORAGE
SAN deployments have been driven by an insatiable demand for storage and the user benefits delivered by networked storage. These benefits include efficient storage. These benefits include efficient storage utilization through storage consolidation, the ability to manage more storage capacity, rapid deployment of new storage, higher availability, and faster backup and restore operations.
With the recent development of the iSCSI protocol and silicon-based TCP/IP offload engines, SANs based TCP/IP networks are now possible. The IP networking infrastructure includes multi-gigabit networks, sophisticated bandwidth allocation and network management tools, and the ubiquitous reach of IP and Ethernet. These factors together enable new IP Storage solutions, using iSCSI protocol.
Fig.3 Factors enabling iSCSI storage
LEVERAGING THE BEST FROM STORAGE AND NETWORKING
iSCSI builds on the two most widely used protocols from the storage and the networking worlds. From the storage side, the iSCSI used the SCSI command set, the core storage commands used throughout all storage configurations.
On the networking side, iSCSI uses IP and Ethernet, which are the basis for the most corporate networks, and are increasingly being used for metropolitan and wide area networking as well. With almost eighty years of research, development and integration, IP networks provide the almost in manageability, interoperability and cost-effectiveness.
Fig.4 Leveraging SCSI and IP for Storage Fig.5 iSCSI Protocol
Network Stack
Fig.4 shows how SCSI is mapped to TCP/IP through the discs layer, freeing SCSI from its parallel bus structure.
Fig.5 shows a simplified version of a protocol stack including iSCSI. Use of the standard SCSI command set facilities interoperability with existing operating system and applications that lay above that layer. Additionally, use of a standard TCP/IP network provides universal reach to the global IP infrastructure.
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