External DAS: Take the good with the bad

External direct-attached storage offers advantages in terms of price, capacity and flexibility, but there are some downsides that administrators should be aware of.

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The term direct-attached storage, or DAS, doesn't necessarily mean disks inside the physical server. Many vendors design external DAS enclosures that can add capacity to a maxed-out desktop or can support a small server.

Although most DAS enclosures offer smaller overall capacities than SAN and NAS appliances (and accordingly lower prices), some of them offer features as elaborate as any SAN array. Here's a look at the pros and cons to external DAS.

  • You can put a lot more storage capacity into an external enclosure than you can cram into the server itself. Storage capacities range from 2 terabytes on units designed to add hot swappable storage to desktops, such as the Nobilis 1004-DAS from Equus Computer Systems (http://www.equuscs.com/) to units that will support 6 terabytes or more. Enclosures are widely available to support anywhere from two to 14 drives. The 1004-DAS lets you add hot swappable SATA storage to an existing or new system. It offers four hot swappable SATA2 bays, enabling up to two terabytes of DAS.
  • It is almost always faster and easier to replace drives in an external enclosure than in the server itself. A drive-only enclosure is simpler than a system unit, which includes the motherboard and interface cards. In addition, some enclosures, such as those from CRU-DataPort, (http://www.cru-dataport.com/) are designed to take self-contained drives, which can be removed from the enclosure for security purposes or to transfer data to another system.
  • Most of these enclosures can support a variety of RAID configurations, including RAID 0, RAID 1, RAID 10, RAID 5 and RAID 50. Many come with hot-swappable drives and dual power supplies for additional reliability. They also offer a choice of SATA and varieties of SCSI, allowing admimistrators to select the right speed and cost for their needs.

What are the downsides to external DAS?

  • An external enclosure takes up additional space -- space that may not be available in desktop installations. Usually, the enclosure is about half the size of the system unit, although the footprint on the desktop is about the same.
  • An external drive enclosure costs more than mounting the drives internally. Although DAS enclosures are cheaper than SAN or NAS devices, they still represent an additional investment. In general, when configuring a system, it makes sense to put one or two drives in the system box and only go to an external enclosure if you need more than two drives. Two-drive enclosures come into their own when you want to add storage capacity to an existing system.
  • While internal drives can be plugged directly into the motherboard, external drive enclosures usually require a separate interface card. The complexity hardly compares with, say, hooking up to a SAN, but it is still a consideration.

Microsoft discusses the pros and cons of external drive enclosures in an article entitled Enterprise Design for DAS.

About the author: Rick Cook specializes in writing about issues related to storage and storage management.

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This was first published in September 2006

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