Direct Attached Storage
Encyclopedia
Direct-attached storage (DAS) refers to a digital storage system
directly attached to a server
or workstation
, without a storage network
in between. It is a retronym
, mainly used to differentiate non-networked
storage from SAN
and NAS
.
(for example enclosure
s holding a number of hard disk drives) connected directly to a computer through a host bus adapter (HBA). Between those two points there is no network device (like hub, switch, or router), and this is the main characteristic of DAS.
The main protocols used for DAS connections are ATA
, SATA
, eSATA, SCSI
, SAS
, and Fibre Channel
.
A DAS device can be shared between multiple computers, as long as it provides multiple interfaces (ports) that allow concurrent and direct access. This way it can be used in computer clusters. In fact, most SAN-attachable storage devices or NAS devices can be easily used as DAS devices – all that is needed is to disconnect their ports from the data network and connect one or more ports directly to a computer (with a plain one-to-one cable).
More advanced DAS devices, like SAN and NAS, can offer fault-tolerant design
in many areas: controller redundancy, cooling redundancy, and storage fault tolerance patterns known as RAID
. Some DAS systems provide embedded disk array controller
s to offload RAID processing from the server's host bus adaptor (HBA). Basic DAS devices do not have such features.
A DAS can, like SAN or NAS, enable storage
capacity extension, while keeping high data
bandwidth
and access rate.
share data or unused resources with other servers. Both NAS
(network-attached storage) and SAN
(storage area network) architectures attempt to address this, but introduce some new issues as well, such as higher initial cost, manageability, security, and contention for resources.
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
directly attached to a server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
or workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
, without a storage network
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
in between. It is a retronym
Retronym
A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...
, mainly used to differentiate non-networked
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
storage from SAN
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
and NAS
Network-attached storage
Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...
.
Features
A typical DAS system is made of a data storage deviceData storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
(for example enclosure
Disk enclosure
A disk enclosure is essentially a specialized chassis designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the data sent across their native data bus into a...
s holding a number of hard disk drives) connected directly to a computer through a host bus adapter (HBA). Between those two points there is no network device (like hub, switch, or router), and this is the main characteristic of DAS.
The main protocols used for DAS connections are ATA
AT Attachment
Parallel ATA , originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITS committee...
, SATA
Sata
Sata is a traditional dish from the Malaysian state of Terengganu, consisting of spiced fish meat wrapped in banana leaves and cooked on a grill.It is a type of Malaysian fish cake, or otak-otak...
, eSATA, SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
, SAS
Serial Attached SCSI
Serial Attached SCSI is a computer bus used to move data to and from computer storage devices such as hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in data centers and workstations,...
, and Fibre Channel
Fibre Channel
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
.
Storage features common to SAN, DAS and NAS
Most functions found in modern storage do not depend on whether the storage is attached directly to servers (DAS), or via a network (SAN and NAS).A DAS device can be shared between multiple computers, as long as it provides multiple interfaces (ports) that allow concurrent and direct access. This way it can be used in computer clusters. In fact, most SAN-attachable storage devices or NAS devices can be easily used as DAS devices – all that is needed is to disconnect their ports from the data network and connect one or more ports directly to a computer (with a plain one-to-one cable).
More advanced DAS devices, like SAN and NAS, can offer fault-tolerant design
Fault-tolerant design
In engineering, fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level , rather than failing completely, when some part of the system fails...
in many areas: controller redundancy, cooling redundancy, and storage fault tolerance patterns known as RAID
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...
. Some DAS systems provide embedded disk array controller
Disk array controller
A disk array controller is a device which manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache.A disk array controller name is...
s to offload RAID processing from the server's host bus adaptor (HBA). Basic DAS devices do not have such features.
A DAS can, like SAN or NAS, enable storage
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
capacity extension, while keeping high data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
and access rate.
Disadvantages
DAS has been referred to as "Islands of Information. The disadvantages of DAS include its inability toshare data or unused resources with other servers. Both NAS
Network-attached storage
Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...
(network-attached storage) and SAN
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
(storage area network) architectures attempt to address this, but introduce some new issues as well, such as higher initial cost, manageability, security, and contention for resources.
See also
- RAID
- JBOD
- Storage area networkStorage area networkA storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
- Network-attached storageNetwork-attached storageNetwork-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...
- Storage Management Initiative SpecificationSMI-SSMI-S, or the Storage Management Initiative – Specification, is a storage standard developed and maintained by the Storage Networking Industry Association . It has also been ratified as an ISO standard...
- Storage Networking Industry AssociationStorage Networking Industry AssociationAn association of producers and consumers of storage networking products, whose goal is to further storage networking technology and applications.The Storage Networking Industry Association, or SNIA, was incorporated in December, 1997, and is a registered 501 non-profit trade association...