When should I use DAS instead of NAS?

Why should storage be easy to use?

The need to add storage capacity appeals to more than those who are IT professionals. Small Offices and Home Offices (SOHO) need to add server class storage without the luxury of using IT services. This SOHO market is large, and the need for expanding storage capacity is spreading into the Home Entertainment or Consumer spaces as well. It is vital to these segments, for the installation and maintenance of the storage server to be very easy. They need to come preformatted and striped, and ready to go for the most common usage models. This large customer base can only be served if these low-end storage expansion severs hide the enterprise class technology, that makes them work. These non-enterprise consumers need protected storage and not just capacity alone or performance alone. Typically there are two system solution choices to serve this market, 1) Network Attached Storage (NAS), and 2) Direct Attached Storage (DAS). Some advocate NAS for the small sever market and yet the majority of the evidence leads to DAS being a better solution for this market.

What is a NAS solution?

NAS is directly attached to the network media and uses file management over low-cost Ethernet-based networks. Ethernet began as a network media in the 1980s. Typical for the SOHO market, the network peak bandwidths are 10Mbps, 100Mbps, and sometimes 1Gbps (1.25MBps, 12.5MBps, 125MBps). Ethernet is a media and it uses an IP-based protocol, such as TCP/IP generally run on top of Ethernet protocol.

At the system level the storage management uses the Network File System (NFS) or Common Internet File System (CIFS), to allow for sharing the data, and these file systems run on top of the TCP/IP protocol. The NFS file protocol is device-independent and an NFS command reads the first 80 characters from a file, without knowing the location of the data on the device. The NAS box receives an NFS or CIFS request over a network and has an internal processor which translates that request to the proper storage command in ATA or SATA, to access the appropriate storage device.

The NAS controller can provide protection for the data using a redundant array mode, if there is a storage controller. NAS generally costs more than DAS (because of its built-in file sharing intelligence) and the need for the additional NAS controller. NFS is actually supported directly by most operating systems, or is available from many software vendors. Due to the NAS box being attached over the network media, large numbers of users are able to access the same storage device.

NAS installation requires some knowledge of network topology and how IP addresses are provided for the target network. The user must assign an IP address for the NAS box and configure it over the network (set permissions and structuring the drives).

What is a DAS Solution?

DAS is optimized for single PC or server with low initial cost. The storage is directly attached by an I/O cable to the PC or server. The media could be any (i.e., SCSI, 1394, USB or SATA). DAS is like a desktop system with its internal hard disks. The key to DAS architecture is that all the storage is directly controlled by the PC or server, and not mounted remotely on a network node like NAS. The next question for the SOHO user is which bus comes with your primary PC or server. To insure the DAS solution is easy-to-use, host buses already available on the outer skin of the PC or server should considered before adding an HBA upgrade.

Some of the host bus speeds are 12MBps for USB 1.0, 40MBps for UltraSCSI, 50MBps for 1394a, 60MBps for USB 2.0, 80MBps for Ultra2 SCSI, 100MBps for 1394b, 150MBps SATA Gen1, 160MBps Ultra160 SCSI, 300MBps for SATA Gen2, 320MBps Ultra320 SCSI.

If you need more than one DAS, your environment becomes islands of storage. Each server or PC must be managed one at a time in the multi-DAS scenario. The PC's or servers are performance bottlenecks in the topology just like the NAS solutions.

Where does NAS fall down?

NAS become performance bottlenecks, when you compare the NAS peak speed (typically 12.5MBps) with the speed even a single drive is capable of providing (50-60MBps).

* Multiple NAS boxes increase the management complexity with multiple IP locations.

* NAS boxes require IP configuration and system setup.

* Adding the second NAS box introduces the same complexity as with a DAS environment and each NAS box must be managed separately.

* Network media speed is a limiting factor for storage performance (typically 12.5MBps).

* Network traffic loads that are not storage focused can slow a NAS access down.

The main areas where NAS falls down and affects ease-of-use are the complexity of installation and configuration. Before beginning the process you need to first find out whether or not your network has a DHCP server running. A DHCP server could be a server computer running some kind of server operating system or maybe a router for you to split your DSL or Cable broadband IP address. If there is no DHCP server in your LAN, you need to find out if your computers were for some reason assigned static IP address. If all this DHCP and IP talk sounds like a foreign language to an average SOHO customer, then you can see the issue which NAS brings to this market. At this stage, due to frustration, many SOHO customers either consult a network administrator or select an easier to use storage product. Remember, one small incorrect change in the network's IP scheme or DHCP setting can take down your entire network and NAS box.

Where Does DAS fall down?

There is also a limit to how much capacity can be directly attached to each PC or server; due to the number of external ports (this is also true for NAS boxes).

* If host buses do not support hot plug, then a DAS connected to a PC or server is taken offline for expansion or maintenance, all the data attached to it becomes unavailable causing downtime.

* There is no way to manage the separate storage islands as a coherent whole and this is true for multiple NAS boxes.

* DAS can stress your local processor as the data is accessed remotely. The PC or sever must manage the network traffic to the DAS, serving files while handling local applications.

* Multiple DAS boxes increase the management complexity with multiple locations. Adding an HBA card to provide faster host bus access can provide some hurdles in configuration and set-up.

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* The main area where DAS falls down and affects ease-of-use is when the network grows large enough and the complexity of multiple DAS boxes becomes a management burden and the SOHO is no longer small or at home.

When is NAS the best solution?

* Use NAS when performance is not the issue and sheer capacity is more important.

* Use NAS when you have a large network and locations that are separated.

* Use NAS if you understand IT management techniques, DHCP and IP settings.

* Use NAS if you must support various OS types in your SOHO and can only afford one storage box.

* Use NAS if you need to manage your storage from a remote location using a web browser over the network.

* Use NAS if there is no issue to have an enterprise IT infrastructure, and it is easy justify technology investments for the IT budget.

When is DAS the best solution?

* Use DAS when you have a small network with only a few servers or power PC users.

* Use DAS when storage performance and local capacity is critical for the power users.

* Use DAS when you have a small location and are not expecting significant growth. DAS is ideal for localized environments with a single server or a few servers in small businesses or departments and workgroups that do not need to share information over long distances or across an enterprise.

* Use DAS when you want easy-to-use storage without IT configuration issues. Setting up, upgrading, or maintaining a DAS box can be handled by just about anyone with any network experience at all.

* Use DAS if you want to use the most common type of storage expansion, in the form of independent drives, RAID arrays or tape libraries. DAS is the most basic level of storage, in which storage devices are part of the host computer, as with drives, or directly connected to a single server or PC.

* Use DAS because it has the lowest initial cost in comparison to other storage solutions and requires the lowest support or maintenance.

Final Recommendation

If you want performance, NAS at peak levels can only offer 12.5MBps, while DAS can offer 300MBps, using SATA Gen2 host buses. If you want a lower cost storage solution, then DAS is the proper selection. If you want a storage solution that is easy-to-use, then DAS can be used without the need of IT professionals. Clearly, for the SOHO market, the right solution is DAS and not NAS.

Conrad A. Maxwell is CEO of Maxwell Consulting Associates, Inc. (Corona, CA)

www.maxwellca.com