In fact, the term DAS was coined in the 1990s in response to the emergence of new networked storage technologies such as storage area networks (SAN). Earlier, DAS was simply storage, or in IBM parlance, DASD – a direct access storage device.
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DAS systems are generally characterized by high bandwidth so there's no question that a given machine will get the benefit of being directly attached to its storage. When DAS includes multiple storage devices and multiple enclosures, connection to a server is generally accomplished using a host bus adapter (HBA).
Of course, that is not to suggest that DAS has been entirely immune to technological innovation.
In fact, Roger Cox, a research vice president at Gartner says there are in effect two kinds of DAS. One is the truly traditional DAS, with the storage within the server enclosure or in a nearby enclosure. Sometimes the drives involved are simply in a just-a-bunch-of-disks (JBOD) format, though various flavors of RAID are also common. However, he notes, the second kind of DAS has evolved with the times, by leveraging developments in networked storage. Webster says some of these newer DAS developments include the application of SCSI, serial attached SCSI (SAS), serial ATA (SATA) and Fibre Channel drives.
The proverbial $64,000 question remains: Why has DAS persisted in the face of the compelling cost-effectiveness of networked storage? "A lot of storage capacity remains in direct attached environments," says Webster, and many users don't plan to change that any time soon. Although networked storage is in fact generally a more efficient option, Webster says there are good reasons why DAS has hung on in many places. "It boils down to a people to people issue," he says.
In the real world, says Webster, storage administrators have to deal with users and groups of users. Some of these users have long since come around to the value of shared or "multitenancy" storage as exemplified in the networked model but others are still deeply resistant and practically paranoid when it comes to caring for their data.
"There are people out there who see data in mine and theirs terms," says Webster. And, no matter how well the advantages of networked storage and shared resources are articulated they simply refuse to consider anything less than storage reserved for their group or their application alone -- and completely walled off from access by others.
In addition to inertia and innate conservatism, Webster says DAS also "survives" in a few instances within Web 2.0 shops "where for some applications people have set up have dedicated, blade servers with their own DAS." However, says Webster, such instances are relatively rare and usually amount to only a small amount of storage.
For those who must live with a DAS reality, the question of how to make DAS more cost-effective might seem pointless. However, Gartner's Cox says savvy DAS owners have tackled this challenge by borrowing from networked technologies to make increased use of storage tiering. For example, SAS and SATA drives are deployed to take advantage of their respective cost and performance advantages. "There is also a great deal of new functionality coming online in terms of reasonably priced arrays that provide SAN-like choices within a DAS set up."
Is there such as thing as too much DAS for one server? Webster says that's a possibility, since managing and operating devices can be burdensome. On the other hand, a high performance computer coupled with lots of high performance storage can work just fine. "It really depends mostly on the nature and performance of the storage devices themselves," he adds.
But Webster still insists that avoiding networked storage doesn't pay off in the long term. Once you get past the initial cost, he says, adding gigabytes to a SAN is no more expensive than adding it to DAS. "The only way DAS is really cheaper at all is if you are sticking purely to JBODs, with no RAID protection."
But ignoring the question of whether or not it's the best investment strategy, you can still do clever things with DAS. For instance, Webster says some shops with creative database administrators have used NetApp's fabric-attached storage (FAS) systems, connected via dedicated Gigabit Ethernet link to a specific server. "That can give you great performance with many of the practical advantages of a network," he notes, "just don't tell your customers". Cox also says that similar things have been done by DAS users with EMC Clariion or the DS4000 disk array series from IBM Corp.
The bottom line, notes Cox, is that having DAS in your shop doesn't mean you have to stand still. New technologies can yield benefits in reliability, performance, and cost. And those investments can be leveraged down the road if or when a migration to networked storage becomes possible.
Alan Earls is a frequent contributor to SearchSMBStorage.com.