Building iSCSI storage area networks - Storage Management

Computer Technology Review, Nov, 2003 by Eric R. Schott

Although the price of storage hardware has been steadily decreasing, the voracious demand for data (on average, capacity requirements double every 16 months) and the associated administrative and maintenance costs continue to strain budgets and staff. Organizations that primarily use direct-attached storage (DAS) find it especially challenging to manage and scale these storage systems in an efficient and cost-effective manner.

One way of surmounting this challenge is to consolidate storage into a storage area network (SAN), which centralizes storage resources and reduces points of management. Not only does a SAN facilitate scaling and enhance availability, it makes it possible for disparate platforms and applications to share the same storage environment, greatly improving utilization, efficiency, and storage administrator productivity.

Until recently, Fibre Channel (FC) has been the sole means of implementing a SAN. However, FC comes with a significant price tag and often requires capital and resource investments in complicated and proprietary technologies. For applications deemed mission critical, large businesses have been able to justify the high cost of owning Fibre Channel SANs. For many applications like e-mail and databases, FC SANs remains prohibitively expensive in terms of cost and complexity.

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To be viable for a broad range of environments and applications, first and foremost, networked storage must be affordable and easy to install, manage and scale. Support for industry standards is vital to ensuring interoperability across different platforms.

Today, the iSCSI protocol makes it possible to extend the benefits of consolidated storage throughout the industry. Devised by the Internet Engineering Task Force--the same organization that created the TCP/IP standard--the iSCSI protocol lets businesses leverage existing IP skills and infrastructure to create Ethernet-based SANs that deliver the features and performance of Fibre Channel, but at a fraction of the cost and greater simplicity.

How iSCSI Works

The iSCSI protocol is built on SCSI and Ethernet, the dominant industry standards for storage and networking. Utilizing an ordinary IP network, iSCSI transports blocklevel data between an iSCSI initiator on a server and an iSCSI target on a storage device. Authentication methods are used to provide security between initiator and target.

When an iSCSI initiator connects to an iSCSI target, the storage is seen by the server operating system as a local SCSI device that can be formatted as usual. The underlying storage is transparent to applications and operating systems (see Figure 1).

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To access storage in an iSCSI SAN, a server needs only an iSCSI initiator connected to an IP network. An initiator can be an iSCSI driver with a standard network card or a card with a TCP offload engine (TOE), which reduces CPU utilization. Also available are host bus adaptors (HBAs) that offload both TCP and iSCSI.

Interoperability across operating systems, applications and hardware is a key benefit of an iSCSI SAN. By providing access to storage at the block level, iSCSI lets heterogeneous servers and applications share the same storage resources. This capability is analogous to the interaction between Web servers and browsers--devices that work together, regardless of platform.

Using a Gigabit Ethernet network, an iSCSI SAN delivers performance comparable to Fibre Channel. Ethernet's long history of improvement, including recent advances in 10Gb Ethernet, ensures the long-term performance viability of iSCSI and graceful network upgrades. Technologies such as multi-path I/O can also be used to improve availability and increase bandwidth.

Despite its recent appearance in the storage world, the iSCSI protocol is widely accepted in the industry and supported by all the major operating systems, including Windows, Linux, Novell and Solaris. Today, many prominent storage vendors are delivering products based on the iSCSI protocol, including Adaptec, Cisco, Intel, QLogic, and a number of innovative emerging companies.

Common Myths About iSCSI

In reviewing the capabilities of iSCSI, it is important to understand that iSCSI does not:

* Change how an operating system works or perceives storage. Once an iSCSI initiator connects to an iSCSI target, the device appears on the server as a regular SCSI disk.

* Require high overhead on the host. iSCSI's broad connectivity options allows for choice in the host connection to storage. This flexibility allows administrators to choose the most cost- effective connection, and the easy ability to change connection method as needed.

* Modify how storage devices work. Using iSCSI does not affect the features, performance, expansion, or price of storage arrays.

* Alter the cost of storage devices. Implementations of iSCSI storage arrays, like any other storage device, can be cheap or expensive, feature-poor or feature-rich.

Comparing SAN Cost Factors

What has prevented the widespread use of SANs to date is the cost and complexity of deploying and maintaining the environment. Administrators must install and manage many individual hardware and software components, some of which may not work in a heterogeneous environment or may require significant customization. The burden of supporting and upgrading disparate elements is beyond the means of many organizations.