DAS is storage directly connected to a server instead of networked through Fibre Channel or Ethernet switches. DAS storage can be inside the server or an external drive or group of drives attached to a server via SATA, SAS or parallel ATA or SCSI interfaces.
Networked storage, in the form of SANs and NAS, have its obvious advantage, but organizations often turn to DAS as a low-cost alternative for certain types of applications. For instance, Microsoft recommends DAS for Exchange to lower cost and to simplify management.
Other emerging technologies, such as SAS drives, blade servers and virtual servers, make DAS more viable than it used to be by improving performance, lowering cost or necessitating shared storage. And while it appeals mostly to small and medium-sized businesses (SMBs) because of cost, analyst Greg Schulz of the StorageIO Group says there's more DAS than you would expect in larger shops.
"If you believe everything you read, there's no such thing as DAS in the enterprise," Schulz says. "The reality is, there is a lot of DAS in the enterprise. There are a lot of application-specific servers and file servers that still have DAS."
The most common form of DAS comes from server/storage vendors, such as Hewlett-Packard and Dell. More storage-specific DAS products include IBM's DS3000, HP's MSA 2000 and EMC's Clariion AX4, usually pitched as entry-level configurations.
Schulz says there are also "hidden forms of DAS" that are "more prevalent than people think. Some deduplication appliances are effectively served with DAS attached. EMC Celera is a NAS head with Clariion-attached DAS."
Newfangled clustered systems, such as HP's 9100 Extreme Data Storage System and EMC's Atmos, are also DAS with intelligent software.
Jonathan Smith, CEO of IT outsourcer ITonCommand, says the two Denver data centers his company uses to host customer data have no Fibre Channel storage -- just iSCSI SANs and DAS.
"DAS is a good low-cost option," Smith says. "It gives us a lot of drive space, and it's real fast. We might have a client with a lot of data who doesn't want to pay for redundancy or clients that are database heavy. We use DAS for them because iSCSI isn't fast enough. Big databases that are 30 gig to 50 gig aren't written optimally for SANs."
Smith estimates it would cost his customers twice as much to put data on a Fibre Channel SAN as on the HP ProLiant and blade servers ITonCommand uses. "Not everybody wants to pay for a SAN," he says. "A company might have a terabyte of data and is not going to pay to put that on a SAN at $20 a gig or whatever it may cost. "
Lee Johns, director of marketing for entry-level storage and storage blades at HP, agrees that cost is what keeps DAS alive in the enterprise. "A SAN is a great thing, but a centralized SAN is an expensive proposition," he says. "What we hear from people who buy servers and virtual machines is the expense of going to a SAN is just too much for them. DAS is typically for someone in a server environment who wants shared storage. Maybe it's a simple VMware environment, or maybe they want to use DAS for Exchange."
This was first published in December 2008