Understanding Storage Terminology

DAS, SAN, NAS, NAS Gateways, Fibre Channel, and iSCSI

DAS = Direct Attached Storage

Direct Attached Storage (DAS), usually disk or tape, is directly attached by a cable to the computer processor. The hard disk drive inside a PC or a tape drive attached to a single server are simple types of DAS. I/O requests, also called protocols or commands, access devices directly. This is the simplest form of storage to deploy, but severely lacks in terms of reach, scalability, and performance in a networked environment.

SAN = Storage Area Network

In a Storage Area Network (SAN), storage devices reside on a dedicated network separate from the LAN. Like DAS, I/O requests access devices directly. Today, most SANs use Fibre Channel media, providing an any-to-any connection for processors and storage on that network. Ethernet media using an I/O protocol called iSCSI is emerging in 2001.

SANs can provide better performance and more cost effective storage management for companies with certain data access requirements. A SAN provides a separate network within an enterprise so that the main communications LAN(s) are not used to pass data between devices within the SAN. This translates into better performance.

An example of a typical SAN is the notion of a "fibre cloud" whereby storage devices (disk drives, tape drives, autoloaders, and libraries) and servers can pass data back and forth seamlessly. Furthermore, the benefit of SAN is a more efficient and thereby economical use of storage resources by the various application servers. Storage consolidation to a subsystem architecture (like ESS) within a SAN can save an organization significant dollars as compared to adding storage to separate application servers.

NAS = Network Attached Storage

With Networked Attached Storage (NAS), a storage device called an appliance, which is usually an integrated processor plus disk storage, is attached to a TCP/I P-based network (LAN or WAN), and accessed using specialized file access/file sharing protocols. File requests received by a NAS are translated by the internal processor to device requests. With NAS appliances, organizations do not need to invest in a SAN, but they also do not realize the benefits of a SAN. However, NAS has benefits of its own, usually in terms of lower costs and faster, simpler deployment.

NAS Gateway

A NAS gateway is a way to incorporate DAS with NAS, or SAN and NAS. A NAS gateway is a NAS device without integrated storage (i.e. just the NAS processor). Instead, the NAS device connects externally to storage by direct attachment or by a SAN. For companies that have departmental requirements that would make it attractive to tap into a SAN but cannot cost justify the acquisition costs to acquire and install fibre adapters to networked servers, NAS gateways allow local or remote attachment of desktop workstations and small servers so as to have a cost effective access to a SAN.


iSCSI is an I/O protocol whereby storage is attached to a TCP/IP-based network, and is accessed by block I/O SCSI commands. iSCSI could be direct attached (DAS) or network attached via a SAN. Key point: DAS and SAN are approaches to connectivity, while NAS is really a type of device whereby either DAS or SAN is deployed. iSCSI is a protocol, and in a SAN is an alternative to Fibre Channel.

Tivoli SANergy

SANergy is software from IBM and Tivoli that provides NAS-like file sharing, with data sent over the SAN rather than the LAN for improved performance. (IBM NAS gateways also include SANergy function.) Some in the industry are calling SANergy and similar facilities SAFS - SAN Attached File Systems. SANergy has attributes of NAS and SAN, with additional flexibility. SANergy supports the NFS and CIFS protocols, but allows the installation to use virtually any disk storage they want (Fibre Channel, iSCSI, parallel SCSI, and SSA storage will all work.).

Which Approach Do I Choose?

Each type of approach has its strengths and shortcomings, and which one is right for a given customer is based upon a variety of factors. But why are there so many forms of storage networking? For one, new technologies emerge and evolve, but customers cannot justify replacing their investments in previous technologies overnight. Furthermore, no single storage networking approach solves all problems or optimizes all variables. There are trade-offs in cost, ease of management, performance, distance, maturity, and other variables. For the foreseeable future, multiple storage network alternatives will coexist, often within the same organization.

The benefits of the major types of processor-to-storage connectivity can be briefly summarized as follows:

  • DAS is optimized for single, isolated processors and low initial cost
  • SAN is optimized for performance and scalability. Some of the major potential benefits include support for high-speed Fibre Channel media which is optimized for storage traffic, managing multiple disk and tape devices as a shared pool with a single point of control, specialized backup facilities that can reduce server and LAN utilization, and wide industry support.
  • NAS is optimized for ease-of-management and file sharing using lower-cost Ethernet-based networks. Installation is relatively quick,and storage capacity is automatically assigned to users on demand.
  • NAS gateways are optimized to provide NAS benefits with more flexibility in selecting the disk storage than offered by a conventional NAS device. Gateways can also protect and enhance the value of installed disk systems.
  • iSCSI, because it is still early in its product evolution cycle, is likely best suited for situations of relatively low I/O activity. In general, an iSCSI SAN likely has a lower cost than a Fibre Channel SAN and can be built more quickly and with fewer new skills than a Fibre Channel SAN. An iSCSI disk device, all else equal, may be lower cost than a NAS appliance since the iSCSI device does not need to support file systems, file sharing protocols, and other facilities often integrated into NAS products.
  • SANergy is optimized for data sharing (like a NAS), but at SAN speeds. Tivoli SANergy is disk vendor-independent, and can be added to an existing SAN to enhance its value.



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