DAS: Direct Attached Storage
What is DAS?
Direct attached storage (DAS) is a device that attaches directly to a server, internally or externally. DAS has several different names - "JBOD" (just a bunch of disks), "RAID Arrays"
and "Disk Arrays." But, they all describe pretty much the same thing - an enclosure that holds hard drives.
DAS is the most common way to increase storage space. But, for some environments it is not the most efficient solution. Since DAS attaches directly to a server, the server must process all requests for data stored by the DAS. All of these requests may increase server demands and may slow its ability to efficiently provide other services.
A DAS system increases a server's storage capacity while maintaining high data bandwidth and access rates. The most common DAS systems consisit of one or more enclosures holding multiple hard drives and one or more controllers. The main protocols used in DAS are SCSI, SAS, and Fibre Channel. The connection with the server or the workstation is made through an HBA (host bus adapter).
Entry level DAS systems usually consist of an enclosure without active components such as controllers, so access logic and fault tolerance patterns are provided through the server HBA. Middle and top range DAS systems provide embedded controllers. And advanced DAS systems typically offer fault tolerances at every level with controller redundancy, cooling redundancy and RAID. RAID management is off-loaded and simple non-RAID HBAs can be used, lowering costs. DAS controllers enable shared storage access, which permits multiple servers (usually no more than four) to access the same logical storage unit, a feature that is mainly used for clustering. Top range DAS systems share similarities with entry level SAN systems.
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What to remember about DAS
- DAS attaches directly to a server.
- DAS is a chassis (enclosure) that holds hard drives, and hard drives can continue to be added.
- DAS is like an electronic storeroom with limited doors. There are limited ways in (through the connected servers).
Why DAS?
- Low acquisition cost - DAS is considered less expensive than other solutions such as SAN and NAS. But, its management, flexibility and expansion may be limited, increasing total cost of ownership (TCO).
- Future migration opportunity - Some DAS devices can be migrated into a SAN (storage area network) by adding a Fibre Channel interface.
Do you need DAS?
You should choose DAS if you need additional storage for a small number
of servers.
Limitations of DAS
DAS has been referred to as an "Island of Information." Disadvantages of DAS include the inability to share data or unused resources with other servers. Here are some other things to keep in mind when considering a DAS:
- DAS requires a single server interface, which increases server load if you want to share files.
- Since a SCSI interface is frequently used, the DAS device has to be physically close to the server (no more than 12 meters (36 feet)). Currently, Fibre Channel connections are being used to eliminate this restriction on units that support Fibre Channel.
- Limited scalability may be an issue because new drivers must be added to increase storage capacity, and this causes network downtime.
- Organizations that choose DAS will find it necessary to begin using the IP network (switches and hubs) to share data. This increases the load on the system network, which begins to bog down the corporate network.
- To back up DAS, the server either has to have a tape drive installed in it or the backup is done using the network. These backups may cause a network to slow down.
If you need help building the right solution, contact our Network Storage Specialists or call your Zones Account Executive.
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