iSCSI � What Does It Mean for Your Storage Network? Page 3 -

iSCSI � What Does It Mean for Your Storage Network? Page 3

Expanding iSCSI Storage Networks to Metro and Wide Area Networks

Enabling storage over long distances is essential to remote site backup or implementing robust disaster recovery applications. The rapid adoption and expansion of IP data on the Internet has proven the viability of using IP across long distance wide area networks. Although it is expected that initial deployments of iSCSI will use private networks, with the use of IP's security infrastructure (such as Secure Internet Protocol (IPSec) and Secure Sockets Layer (SSL) to provide authentication and privacy), it will be possible to use public networks for wide area iSCSI storage traffic as well.

The 10 Gigabit Ethernet provides the necessary network links, reliability, and bandwidth for carrying large amounts of storage data over both private and public IP networks covering varying distances. To meet the distance objectives of metropolitan area networks/wide area networks (MAN/WAN) storage deployments, the IEEE 802.3ae Task Force specifies the 1550nm serial transceiver to maintain the support of 40 Kilometer links over a single mode fiber that Gigabit Ethernet has already been successful in deploying. The 10GbE also specifies a variety of optical transceivers that support distances including 300 and 10,000 meters over single and multi-mode fiber. The 10GbE standard additionally specifies a WAN Physical Layer Device (PHY) to facilitate compatibility with the existing WAN network. The WAN PHY supports the same distances and optical transceivers as the LAN PHY specification. The 10 Gigabit Ethernet WAN PHY supports connections to existing and future installations of Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) circuit-switched telephony access equipment by including a simplified SONET/SDH framer. The ubiquity of Ethernet and provisions made in the 10GigE Standard, make 10GbE the ideal choice for supporting iSCSI storage networks in metro- and wide area networks.

So, what in simple terms will people need to implement an iSCSI based SAN? Lets take a quick look.

Implementing an iSCSI-based SAN

Storage Area Networks (SANs) are growing rapidly because they solve a problem. That problem is the manageability of large and ever increasing amounts of disk storage. When storage is directly attached to the computer using it through a high-speed cable, the storage device can meet the needs of that computer very well. When computers are networked together (each computer with its own attached storage and total storage), resource management is difficult or impossible to achieve. The SAN approach offers many storage management advantages, including the ability to:
  • Backup without degrading performance for the network users.
  • Balance the load to each physical device.
  • Easily grow the size of the storage pool.
  • Easily share backup devices.
  • Eliminate downtime.
  • Manage your computing resource separately from your storage resource.
  • Manage your storage resource easily and efficiently.
  • Share a pool of storage.

As previously explained, the SCSI and Ethernet standards are each mature, stable, ubiquitous and interoperable. These qualities provide a solid foundation on which to implement a SAN. iSCSI is the emerging Internet Engineering Task Force (IETF) specification that defines how SCSI and Ethernet work together to perform SCSI data transfers across TCP/IP networks. It allows the tried and true block storage SCSI commands to be carried by the standard TCP/IP protocols over the Ethernet wire.

Until recently, Ethernet was too slow to compare to the performance of direct attach storage. With the advent of 1 Gigabit and 10 Gigabit Ethernet, an Ethernet network is now fast enough to use for storage applications. So, why would you want to have a different network infrastructure for SANs than you do for the LANs, MANs and WANs that make up the Internet and your company's own networking? Lets take a look.

Connecting to the Ethernet Fabric

Basically, iSCSI is composed of initiators and targets that will be used at both ends of the Ethernet wire. The iSCSI initiator connects the computer that initiates the request for data stored on the disk with the Ethernet wire. The iSCSI target connects the Ethernet wire with the storage device, whether it be an array of disks or perhaps a tape device. The initiator encapsulates the SCSI commands and data, so that they can be sent over TCP/IP; and, the target extracts the commands and data at the storage device end of the wire.

As with any networking protocol, the software that implements the iSCSI over the TCP/IP approach, is thought of as having layers. One or more of these software layers can be run on the hosting computer, on a coprocessor that lives on a plug-in board, or some of the software function may be implemented in a chip. The approach used determines the cost and performance of the SAN connections. The best approach may be application specific, and your options may depend on the operating system in use on your host computer. Nevertheless, while the potential benefits of a single network infrastructure for storage and networking are many, and will reduce the total cost of ownership of a SAN, there is still work to be done to make this option a practical reality.

Finally, how will an iSCSI based SAN compare in terms of performance and cost to Fibre Channel? Let's take a very brief look.

Fibre Channel Limitations

The differences in an iSCSI based SAN (as compared in terms of performance and cost) to Fibre Channel, is in the limitations of Fibre Channel itself. In other words, while Fibre Channel is a high performance transmission technology optimized for the same block storage format that storage devices use, it does have drawbacks:

Total Cost of Ownership (TCO)

The Total Cost of Ownership (TCO) for operating a Fibre Channel SAN, while lower than the DAS model, is still high. Since organizations vary widely in their storage needs, it is difficult to develop a set of assumptions for generating average cost figures. Still, Fibre Channel is a fairly new technology and many IT staffs have limited, Fibre Channel expertise. Finding the necessary specialized personnel can be challenging and training is often not readily available. As a result, installing and maintaining a Fibre Channel network is typically difficult and expensive.

Operating Distance

Finally, although the theoretical limit for Fibre Channel is 10km, individual multi-mode fiber links used in Fibre Channel SANs may have a practical limitation of 250 to 500 meters. The storage ecosystem is evolving to where large organizations often have SANs located far from the LAN, to provide geographical redundancy as part of disaster planning. This means even 10km may be increasingly inadequate.

Summary And Conclusions

Organizations with changing data requirements, especially those requiring data storage security or disaster recovery, will benefit most from the introduction of IP storage and iSCSI. As performance increases and iSCSI SANs become ubiquitous, they will gradually be integrated into enterprise TCP/IP networks. Distributed intelligent services and automated allocation of storage resources via virtualization, will become an integral part of the future evolution of iSCSI SANs.

Furthermore, iSCSI represents a dramatic shift in the storage networking landscape. iSCSI will expand the market for networked storage by giving IT managers another alternative to direct attached storage that delivers the advantages of networked storage. Today, IP storage networks take advantage of IP networking knowledge in IT departments and use existing network management and tools for LANs, MANs and WANs. Finally, by riding the IP wave of technology development and enhancements (like the introduction of the 10 Gigabit Ethernet), iSCSI provides a logical unified infrastructure development path for enterprises and service providers alike.

» See All Articles by Columnist John Vacca

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