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iSCSI and Serial Attached SCSI: storage technologies for SMBs and remote offices.

The benefits of networked storage in reducing storage costs are well documented. While network attached storage (NAS) systems meet the requirements for applications that access data at a file level, a storage area network (SAN) is required to consolidate the storage resources supporting block I/O applications like databases and Microsoft Exchange. While many small to medium business (SMBs) and remote offices use NAS systems today, the cost and complexity of SANs has limited their use within these organizations.

But new technologies like iSCSI and Serial Attached SCSI can be combined with enhanced storage intelligence to reduce the cost and complexity of installing and managing a SAN. These systems leverage an IT staff's existing knowledge of IP networks, a set of common building blocks, and storage automation to reduce complexity. This simplification promises to enable limited IT staffs to take advantage of SAN technology to better manage growing storage requirements and costs.

Comparing Storage Architectures

Network attached storage (NAS) systems connect to one or more servers through a local area network and provide access to data at the file level. These systems can be thought of as application specific file servers. NAS systems are usually very easy to install and maintain and can provide file services to servers and clients running different operating systems. Because data is handled at a file level, NAS systems must read and write an entire file even if only a small portion of the file is required by an application.


Storage area networks (SANs) connect storage systems to one or more servers through a low latency network dedicated to providing access to storage resources. SANs communicate with storage resources using block I/O commands. Block I/O commands enable an application to read and write only the portion of the data that is required by an application. Applications like databases and Microsoft Exchange require block level access to storage resources. Working at a block level can also improve the performance and efficiency of applications like remote replication and point-in-time copies.

The Differences between Disk Drives

Historically, ATA disk drives were designed to meet the needs of desktop PCs and consumer applications. These applications place a high priority on cost per gigabyte and maximum capacity per drive. In contrast, SCSI drives were developed for use in enterprise-class systems such as workstations, servers, and RAID storage systems. Enterprise systems place a higher value on I/O performance and system availability than on cost per gigabyte and maximum and capacity.

Leveraging the economies of scale associated with desktop PC and consumer applications, ATA disk drives have experienced significant growth in capacity while sustaining remarkable reductions in average cost per gigabyte. This drastic reduction in the cost for capacity has spawned the development of new classes of applications called nearline and midline storage that require very high capacity at a lower cost, but do not require high I/O performance.

Nearline storage is used to enhance the backup, restore and archival processes. Disk-to-disk backup and disk-to-disk-to-tape archiving are examples of nearline applications. One of the primary characteristics of nearline storage is that the data stored is not online for use by end-user applications.

Applications like transactional processing and decision support systems use SCSI or Fibre Channel disks to store data where response time matters. These applications require dynamic online storage capable of continuously servicing both write and read requests. Midline storage represents another tier of storage for business critical applications. It takes advantage of the superior capacity and cost of ATA disk drives to more cost-effectively keep less-frequently accessed data online. Data stored on midline systems is not accessed as frequently as data stored online for mission-critical applications. Therefore it does not require the high I/O capability of dynamic online storage. Midline storage enables organizations to make more data accessible in order to increase customer service levels or improve business decision-making.

Serial Attached SCSI is a new enterprise disk interface developed as a replacement to parallel SCSI to improve storage system performance, scalability, and availability. Serial Attached SCSI controllers support both dual-port, high-performance Serial Attached SCSI drives and high-capacity, lower-cost Serial ATA drives. With Serial Attached SCSI, IT managers have the flexibility to purchase servers and storage systems that can be configured with either Serial ATA or Serial Attached SCSI disk drives or both, enabling the use of high-performance and low-cost disks in the same storage system.

This flexibility enables IT managers to use the same system for both mission-critical and business-critical applications, and reduces data center complexity by standardizing on one storage system for all applications. It also provides investment protection by letting IT managers redeploy storage systems to meet new requirements simply by reconfiguring existing systems.

Reducing Complexity with Intelligent Storage Systems

iSCSI reduces SAN complexity by leveraging a LAN manager's knowledge of IP networks and by utilizing a common set of Ethernet switches as building blocks for both SANs and LANs. Serial Attached SCSI will also lower complexity by providing a common storage platform for mission critical and business critical storage. Automating the tasks required to install and manage a storage system within a SAN can reduce complexity even further. Intelligent storage systems can automate many of the common storage management tasks. This automation, combined with the benefits of iSCSI and SAS, will go a long way in helping the limited IT staff within SMBs and remote offices take on the additional responsibility of SAN management.

A new class of storage arrays based on iSCSI technology uses embedded intelligence to automatically configure, manage, and load balance data across arrays without user intervention. Once configured, the array's intelligence automatically sets up RAID configurations, creates volumes, and sets reserves for snapshots. Remote offices benefit from the array's ability to securely auto-replicate to a site hundreds of miles away over a standard IP network.

Figure 1 shows an example of a typical storage array installation. Intelligent arrays can support a wide range of applications and operating systems, bringing the benefits of centralized management, automated provisioning, and ease of operation and expansion both inside and outside of the data center to SMBs and remote offices.

The first generation of intelligent storage arrays leverages the capacity and cost-effectiveness of SATA disk drives with support for. Serial Attached SCSI expected in early 2006. These new arrays can automatically create hierarchical pools of storage based on data use patterns in the applications they serve. The array was designed to automatically migrate frequently accessed online data to high-performance SAS drives and less-frequently accessed midline data to higher-capacity, lower-cost SATA drives. Lower-cost SATA drives can also be used to configure the array to support nearline data-protection applications.

The Opportunity for SANs within SMBs

The total cost of ownership for a SAN can be less than half of direct attached storage as shown in a report by McKinsey and Company and Merrill Lynch's Technology Research Group. Lower costs are achieved through better disk utilization, reduced requirements for tape drives, and centralized management of storage resources.

In direct attached storage configurations, data archiving is typically performed by either adding a tape drive to each server to perform backup and restore or by copying data across the LAN from each server to a tape library. As the number of servers attached to the network increases, so do acquisition costs for adding tape drives, manpower requirements for performing daily or weekly backups, and traffic on the LAN.

Reducing SAN Costs and Complexity with iSCSI and Serial Attached SCSI

Developing or hiring the people to manage a Fibre Channel SAN is still a significant concern. Many SMBs cannot afford to dedicate people solely to SAN management. At these companies, the people responsible for supporting the LAN will also manage the SAN. A Fibre Channel SAN can also represent a major financial investment. Those SMBs that invest in SAN technology view it as a long-term investment to support growing storage requirements and control storage management costs.

iSCSI utilizes industry-standard Ethernet switches to lower acquisition costs and reduce the learning curve required to install and manage a SAN. With iSCSI, the IT staff at an SMB can standardize on a core group of Ethernet switches to meet both LAN and SAN requirements. Learning curves are reduced by leveraging an IT group's existing knowledge of IP networks and their understanding of their preferred LAN supplier's products. Utilizing a common set of switches also means that an Ethernet switch dedicated to a SAN application today can be redeployed to support LAN requirements tomorrow and vice versa. This versatility provides a level of investment protection for SAN hardware purchases not provided by Fibre Channel SANs.

Storage systems based on Serial Attached SCSI can support online mission-critical and midline and nearline business-critical applications. As with SANs based on iSCSI, IT staff can standardize on one device to meet multiple requirements. Learning curves are reduced by utilizing a common storage system to meet all requirements. A storage system deployed today to meet the requirements of a business-critical application can be reconfigured with new disk drives in the future to meet the requirements of a mission-critical online application. The benefits of iSCSI and Serial Attached SCSI can be combined to create a SAN that provides the flexibility and adaptability SMBs seek when evaluating SAN investments.

Storage systems based on Serial Attached SCSI are expected to cost less than systems based on Fibre Channel due to simpler protocol and higher economies of scale. Like SAS systems, iSCSI also benefits from economies of sdale. IDC forecasts show port shipments for gigabit Ethernet switches are expected to be about an order of magnitude larger than those for Fibre Channel switches.

While Fibre Channel will continue to find a home in high-performance applications, lower acquisition and operational costs provide a significant advantage for the entry-level SANs most often deployed by SMBs and remote offices. Gartner forecasts predict iSCSI will become the dominant SAN attachment technology for entry-level servers by 2008. As iSCSI storage solutions incorporating Serial Attached SCSI emerge, the flexibility and cost advantages of these solutions will create an even more compelling value proposition for SMBs and remote offices looking to invest in SAN technology.

Kevin Gray is senior marketing manager, Maxtor Corporation, Milpitas, CA

John Joseph is vice president of marketing, EqualLogic, Inc., Nashua, NH
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Article Details
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Title Annotation:Connectivity; Small Computer System Interface; small to medium business
Author:Joseph, John
Publication:Computer Technology Review
Geographic Code:1USA
Date:Aug 1, 2005
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