Direct Attached Storage (and devices)

Direct Attached Storage Devices

Definition: DAS is a type of storage that is connected directly to the server which enables quick access to the data but only through the server.

Direct-attached storage (DAS) is a basic level storage in which the host computer holds the storage devices or these can be connected to one server at a time. Networked workstations can access the data only through the host. DAS was the first storage model that gained widespread acceptance and is widely used. Even though networked storage models are gaining popularity, DAS still finds takers because it is easy to deploy and has a low initial cost of deployment. It helps if you have an idea of what your data availability needs are at present and what they will be in the future.

Since clients can access the storage device only through the server, a high percentage of server uptime is critical. A slow server will make storing and retrieving files difficult. Since the server also runs applications, data access may be slow as server bandwidth gets diverted to the applications.

DAS is a cost-effective storage solution for small enterprises though limited in its scalability. It is ideal for setups that rely on localized file sharing and there is no need to transfer files over long distances. Enterprises that begin with DAS but later shift to networked solutions can use DAS to store less critical data.

DAS configuration can be achieved in two ways, one is where the disk is integral to the server and in the second design option the disk is installed in a separate enclosure but attached to the server.

A single enclosure DAS offers some advantages – these include an easy to manage connection that can be managed with minimal skills. This is because the cabling is an integral part of the cabinet with the server. SCSI bus cables do not cost much and therefore logistical planning and administrative overhead costs are kept low. Since the storage is localized, it helps in optimizing performance. DAS is a general-purpose solution for all types of storage processing.

The drawbacks of a single enclosure DAS design include poor scalability and limited disk capacity. This means that DAS cannot be used as the only storage medium for an enterprise environment. Poor scalability adds to the complexities in managing the storage environment. DAS does not allow for good management practices where a single data repository image is maintained. DAS does not provide the uptime or security that is associated with a SAN or NAS configuration. Disk consolidation with DAS is not feasible.

Multiple external enclosure DAS systems are ones where the DAS can be configured with external storage accessed through an input/output card. SCSI interfaces provide extended device management capacity and thus are useful for accessing external storage cabinets. Modern external enclosures offer high throughput rates and redundancy of power to ensure high availability.

A multiple external enclosure DAS design offers the advantage of speedier recovery in case complete server hardware takes place. Storage capacity is in terabytes and greater than the internal capacity of a computer.

On the flip side, a multiple external enclosure DAS adds to the complexity of management; it is more expensive than an internal solution and has greater space requirements. When setting up DAS, the following aspects regarding hard disks should be taken into consideration – disk capacity, disk I/O, and hard disk connectivity.

With DAS, redundancy is provided at the disk or controller level because with locally attached storage the fault tolerance is taken care of by localized DAS technologies. System-level redundancies cost more and in the event of a server problem the attached storage may be unavailable to users. In order to improve data accessibility the Windows Cluster service can be deployed to provide redundant hosts that share the storage subsystem. RAID configurations also add to the redundancy.

Large-scale DAS deployments can be a little difficult to secure because of the distributed nature of the servers. DAS security includes server security policies and access limitations to the server – both physical and over a network. DAS hosted on Windows servers can be made secure by using group policies. DAS scores well on the manageability front so long as scalability is not an issue. Backup and recovery of DAS storage can be done over LAN; but this adds to the LAN traffic and can slow down applications. A solution is to add another network to be used solely for backup and recovery but such a solution adds to the management complexity and may not be adequate for very large databases.

In terms of performance DAS storage delivers well because the processor and disk are situated close to each other. Any effort to scale DAS can result in performance levels falling because the storage and applications share the same set of resources. Unlike NAS and SAN which use dedicated resources for storage processing, DAS affects the LAN passage because of storage-related traffic.

DAS does not lend itself easily to consolidation because of the manner in which it is designed where each DAS unit operates independently.

Interoperability of DAS with NAS and SAN is good. In a given storage environment, the interoperability of the hardware devices is determined by the hardware and supportability matrix developed by the hardware vendors.