iSCSI: Accelerating the transition to network storage

SANDEEP DUTTA, director, Strategic Partnerships and Marketing, Network Appliance, India, describes the need for IP Storage and thereby iSCSI

OVER the past decade, the world has seen enormous increases in the amount of data storage required. This has been driven by a pervasive global economy, e-commerce, e-mail, and the digitisation of media and information. Researchers estimate that there are now several million terabytes of online information, an amount that continues to grow exponentially with every e-mail message sent, every Web and MP3 file posted, and every online transaction executed. As a result, IT executives are increasingly looking to new data storage solutions to manage their growing storage needs.

Today’s technology market offers two broad options for storing data: direct attached storage (DAS) and networked storage. Networked storage in turn consists of two sub-options: storage area networks (SAN) and network attached storage (NAS).

More than one-third of the storage currrently sold is networked storage, a fraction that analysts expect to grow to two-thirds over the next few years. Networked storage architectures separate servers from storage, and can offload much of the data management from the server. To understand the anticipated shift from DAS to networked storage, it is necessary to first discuss the different architectures in greater detail.

Direct Attached Storage

In its simplest form, direct attached storage consists of a disk drive attached directly to a server. Data is transferred using SCSI (Small Computer System Interface) commands, the most common means of I/O communication between a computer and a hard drive. In most enterprise implementations, however, the storage is a shelf of disks, a disk subsystem with integrated RAID protection, or a higher-end storage array providing additional data protection.

Direct attached storage works well in environments with individual or a limited number of servers, but if there are dozens of servers and significant data growth, the situation rapidly becomes unmanageable. The storage for each server has to be managed separately. It can’t be shared. Performance is often limited, as is scalability, and the overall efficiency of storage resource allocation tends to be low. The data management needs of today’s enterprise IT environments are typically much better served by a networked storage approach.

Storage Area Networks

In a storage area network a number of servers have shared access to storage. The servers are connected via host bus adapters (HBAs) to a fibre channel switch, which in turn is connected to the fibre channel storage system(s). The servers and storage communicate via the fibre channel protocol suite, which allows SCSI commands to be transmitted via serial connections. The protocol allows for high throughput, transmitting data at 700-800MB/sec in first-generation products and approximately twice that in the second-generation products shipping today.

However, as in the case of direct attached storage, each server has to incur the overhead of running a volume manager and file system to provide data management services to its applications, and these are often different for each server architecture. SAN storage has considerable advantages over direct attached storage through improved scalability, reliability, availability and performance. However, cost and complexity issues have limited their deployment to mission-critical and high-performance applications, typically in ‘glass house’ data centre environments.

Network Attached Storage

Network attached storage was originally designed for data sharing in a LAN environment. This is accomplished by incorporating volume manager and file system capabilities into the storage device.

In a NAS environment, the servers are connected to the storage by a standard Ethernet network, and they use file access protocols like NFS and CIFS to make storage requests. Local file system calls from the clients are redirected to the NAS device, providing shared file storage for all clients. If the clients are desktop systems, the NAS device provides serverless file serving. If the clients are server systems, the NAS device offloads the data management overhead from the servers.

NAS devices deliver the lowest total cost of ownership of any storage approach, but historically they have been viewed as low capacity and low performance when compared with SAN storage.

More recently, the advent of Gigabit Ethernet and enterprise-capable NAS appliances has greatly accelerated the deployment of NAS solutions for enterprise applications in data centre environments, particularly when ease of use is an important issue. However, since some enterprise applications are architected to view storage as a local disk, NAS is not suitable for all applications.

In a typical corporate IT environment, you’ll find applications that require SAN storage and applications that require NAS storage. The industry is starting to respond to this with unified storage systems which can support both SAN and NAS protocols, and fibre channel and Ethernet transports.

The need for IP Storage

Networked storage is a mature and well-understood technology. Many analysts believe that the main impediments slowing the transition from direct attached storage solutions to networked storage solutions are the cost and complexity associated with fibre channel. However, an emerging technology promises to address these issues: Internet SCSI (iSCSI), or SCSI over IP.

What is Gigabit iSCSI?

Internet SCSI (iSCSI) is a new IETF standard protocol for encapsulating SCSI commands into TCP/IP packets and enabling block data transport over IP networks. iSCSI can be used to build SANs using standard Gigabit Ethernet infrastructure.

An iSCSI HBA, or storage NIC, connects storage resources over Ethernet. As a result, core transport layers can be managed using existing network management applications. High-level management activities of the iSCSI protocol—such as permissions, device information, and configuration—are layered over or built into these applications. For this reason, the deployment of robust, inter-operable, enterprise data management solutions for iSCSI devices is expected to occur quickly.

First-generation iSCSI HBA performance is well-suited for the workgroup or departmental storage requirements of medium-sized and large businesses. The availability of TCP/IP offload engines further improves the performance of iSCSI at 1 Gigabit Ethernet, and will allow vendors to scale to 10 Gigabit Ethernet iSCSI in 2004 or 2005.

Inter-operable SANs

By combining SCSI, Ethernet and TCP/IP, Gigabit iSCSI delivers these key advantages:

• Builds on stable and familiar standards since many IT professionals are familiar with the component technologies.
• Creates a SAN with a reduced TCO; installation and maintenance costs are low.
• Provides a high degree of inter-operability; reduces disparate networks and cabling, and uses regular Ethernet switches instead of special fibre channel switches.
• Delivers a solution with no practical distance limitations since IP datagrams can travel over the global IP network.

Who can use it?

iSCSI SANs will initially be most attractive to organisations with the following:

• Distributed IT environment.
• Significant data growth over the past five years.
• Proliferation of Intel architecture servers in divisional, departmental and workgroup environments.
• Business requirement to consolidate data storage and management for these environments to improve operational efficiency, data availability and storage resource management.
• Budget and staffing limitations, which preclude a fibre channel SAN deployment.

Displacing DAS

Organisations with server proliferation and data growth problems in departmental and workgroup data centres will be the first to benefit from the introduction of IP storage and iSCSI as they replace direct attached storage and accelerate the transition to networked storage. As the technology matures and performance improves, iSCSI-based storage solutions will gradually expand to displace direct attached storage in high-end data centres and mission-critical environments, making iSCSI SANs omnipresent.