TCO benefits of NAS versus DAS

A brief look at the total cost of ownership benefits of network-attached storage vs. direct-attached storage.


PAGE TAGIZAD - Procom Technology
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Both network-attached storage (NAS) and storage area networks (SANs) have common objectives, including storage sharing by multiple host or client systems, using network interfaces as opposed to direct-attached storage (DAS) buses, and simplified management to reduce costs.

Networked storage topologies have been steadily gaining on DAS topologies. For example, both SAN and NAS revenues more than doubled last year, and they are expected to triple by 2003, according to Peripheral Concepts, a market research firm in Santa Barbara, CA.

However, revenues do not tell the whole story. IT decision-makers buy storage products because of their value proposition, defined largely in terms of total cost of ownership (TCO) and performance.

Total cost of ownership

TCO analyses assess the costs to acquire, deploy, operate, and manage a technology over time. With DAS, when a company requires more storage capacity, IT administrators install more drives in a server or buy another general-purpose server with attached storage. This has the advantage of a low acquisition cost, but it can be a TCO nightmare. Each new general-purpose server leads to two key cost multipliers: downtime and management complexity.

Deploying server-attached storage often incurs a hefty downtime penalty. First, you have to configure the server with the operating system, storage management applications, and network connections. Next, you have to configure the disk array for host operating system access and for volume sharing across a LAN using network file systems such as NFS or CIFS. This process sometimes requires more than a day for complex storage arrays, resulting in a lack of storage availability for about the same period of time.

Once deployed, DAS typically entails high management costs. Managing server-attached storage configurations requires either more IT staff or a proprietary suite of storage management software, which typically has a high TCO. Consequently, in many companies, storage is poorly managed or not managed at all.

According to the Storage Networking Industry Association, it costs approximately $4 per megabyte per year to manage storage. At a time when companies commonly grow their storage infrastructure into the multi- terabyte range, the per-megabyte management costs associated with server-attached storage can add up to mind-numbing figures. According to Forrester Research, the 100 largest companies in the world will have to manage on average 150TB per company by 2004. That would translate into $400,000 to $1.3 milllion per company per year just to manage that storage.

In the absence of effective management, server-attached storage is prone to unacceptable levels of operational downtime. What-ever the cause, the failure of a server is a period of downtime during which storage is inaccessible.

All of the above contribute to a high TCO for server-attached storage, which provides the basis for the value proposition of networked storage.


NAS addresses the TCO drawbacks of server-attached storage.

  • Cost of acquisition: NAS appliances are essentially disk arrays combined with a kernel operating system optimized for network connectivity and storage. With most NAS servers, the acquisition cost is comparable to a server-attached storage configuration of the same capacity.
  • Cost to deploy: NAS does not require connection to a server HBA. It connects directly to a network. With common IP ser vices such as the Dynamic Name Service (DNS), deployment of NAS can be "plug and play." NAS servers usually come pre-configured to support NFS and CIFS and typically require only a few minutes of configuration. In the most complex instances, additional configuration steps may be required (e.g., to create virtual volumes or to customize RAID configurations). However, these steps are usually performed offline and before device installation.
  • Cost to manage: Most NAS servers feature their own management utilities. Many products can also be monitored and managed via standard SNMP-based network management systems already in use. In short, NAS deployments do not require additional IT personnel skills or new storage management programs.
  • Cost to operate: Without a general-purpose operating system, NAS products have fewer components to fail and fewer reasons to go offline. High-availability configurations enable continuous access to data, even in the event of a disk failure or other disruption. Comparable high-availability solutions in the server-attached storage typically require clustered servers and storage interconnects that can be difficult to maintain and operate and costly to acquire and deploy.

The current trend toward networked storage speaks to the widespread acknowledgement of the need for a solution that scales beyond the confines of server-tethered storage. Such a solution must also address TCO issues.

Page Tagizad is director of product marketing at Procom Technology (www.procom.com) in Irvine, CA.

This article was originally published on July 01, 2001