iSCSI: changing the economics of storage

Internet SCSI (iSCSI) technology is changing the face of networked storage, providing a cost-effective and easy-to-manage alternative to Fibre Channel networks, argues Travis Vigil.

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By  Travis Vigil Published  May 31, 2007

Storage is a critical component of many enterprise infrastructures, and the need to support steadily growing amounts of digital content, provide 24/7 access, and comply with regulatory requirements has only increased the importance of controlling costs and maximising the efficiency of storage resources. The most common solution to these issues is the deployment of a Fibre Channel-based storage area network (SAN), which is typically easier to manage and back up, provides higher utilisation, and offers lower total cost of ownership than internal storage or direct attach storage (DAS) in large environments. However, Fibre Channel SANs also tend to have a higher cost of entry than other types of storage, and these costs combined with a steep management learning curve can place Fibre Channel SANs outside the reach of many small and medium-size enterprises.

For similar reasons, these factors often prevent large enterprises from extending their Fibre Channel SANs to departments and workgroups.

iSCSI has the potential to surpass Fibre Channel in bandwidth over the next few years.

Internet SCSI (iSCSI) can provide the advantages in scalability, availability, and manageability of Fibre Channel but at a lower acquisition cost, helping eliminate this barrier to entry for organisations that want to deploy a SAN without investing in a Fibre Channel infrastructure. Importantly, iSCSI also allows IT staff to work with familiar TCP/IP networks and standard Ethernet components, avoiding the need to learn how to deploy and manage specialised Fibre Channel equipment.

iSCSI storage can be implemented in several ways, including as an alternative to Fibre Channel for enterprises that have not yet implemented a SAN, as a complement to an existing Fibre Channel SAN, and as a complement to network attached storage (NAS) for enterprises that are seeking integrated file- and block-level storage access.

To understand the advantages offered by iSCSI technology, enterprises should understand the strengths and weaknesses of traditional approaches: internal storage, DAS, and networked storage such as NAS and Fibre Channel SANs.

Internal storage: Internal storage is typically simple and inexpensive, but does not scale well. If the server's maximum storage capacity or processing power is reached, administrators must add servers to meet the new requirements, potentially underutilising either their storage or processing resources. Internal storage also offers limited high-availability options because a server failure renders that server's storage unavailable. Because each server's storage must be managed and protected separately, this type of storage can be difficult and time-consuming to administer in large environments.

Direct attach storage: DAS is a logical extension of internal storage, and typically consists of a rack of external hard drives utilised by a single server to expand capacity, although innovations have recently provided the flexibility for several servers to access the same DAS array. Other improvements - such as multiple drive options like Serial ATA (SATA) and Serial Attached SCSI (SAS), enhanced scalability and data availability, and the introduction of applications with built-in replication capabilities like Microsoft Exchange Server 2007 - have further increased the usefulness of DAS. However, because the number of attached hosts is limited, DAS can still create the same sort of management difficulties as internal storage in large environments.

Networked storage: NAS and SANs enable many hosts to share storage resources, helping enhance storage utilisation, simplify management, and reduce total cost of ownership compared with internal storage and DAS.

NAS is primarily used for file sharing, while SANs are typically used for application data. Given that the initial entry cost for SANs is typically much higher than internal storage or DAS, deployment has primarily been restricted to large enterprises with the resources to invest in this type of infrastructure. Fibre Channel SANs can also have a steep learning curve for administrators unfamiliar with their deployment and management.

This is where iSCSI fills a longtime gap in enterprise storage - a networked storage environment that can offer the benefits of a SAN while avoiding the high entry costs and specialised knowledge typically required for Fibre Channel infrastructures. iSCSI enables small and medium-size enterprises to deploy SAN technology that may have been previously out of reach, while enabling large enterprises to create a complementary storage tier for secondary servers or expand their existing networked storage to include departments and workgroups, helping consolidate storage for increased capacity utilisation.

The iSCSI protocol allows SCSI commands to be sent over an Ethernet network, enabling enterprises to build SANs with standard equipment such as Gigabit Ethernet switches and network interface cards (NICs). An iSCSI SAN accesses and transfers data in blocks, with each connected server seeing the remote storage array as a local hard drive. (This is a different implementation from NAS, which abstracts and organises block data so that connected systems see the storage as files.)

Much like Fibre Channel, iSCSI networks use two types of nodes: initiators (the hosts requesting data from storage) and targets (the storage holding that data). Unlike Fibre Channel, administrators can install free initiator software, widely available from sources such as Microsoft and standard Linux OS distributions. iSCSI also offers a variety of connectivity options. Servers can connect to iSCSI SANs using standard Ethernet NICs or, if extremely low CPU utilisation is required, iSCSI host bus adapters (HBAs). Another key technology that benefits iSCSI is the TCP/IP Offload Engine (TOE); this dedicated hardware is responsible for TCP/IP processing and helps avoid using CPU cycles for network traffic. Like iSCSI HBAs, TOEs also help reduce the CPU utilisation of hosts connected to an iSCSI SAN, leaving the CPU available for application processing.

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