A Study On Direct Attached Storage Computer Science Essay

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Direct-attached storage is computer storage that is directly attached to one computer or server and is not, without special support, directly accessible to other ones. The main alternatives to direct-attached storage are network-attached storage (NAS) and the storage area network (SAN).

For an individual computer user, the hard drive is the usual form of direct-attached storage. In an enterprise, providing for storage that can be shared by multiple computers and their users tends to be more efficient and easier to manage.

Examples of Direct Attached Storage:

Direct Attached Storage refers to a range of storage devices that have a single connection to a computer or device. These include:

Network-attached storage (NAS) is hard disk storage that is set up with its own network address rather than being attached to the department computer that is serving applications to a network's workstation users. By removing storage access and its management from the department server, both application programming and files can be served faster because they are not competing for the same processor resources.

The network-attached storage device is attached to a local area network (typically, an Ethernet network) and assigned an IP address. File requests are mapped by the main server to the NAS file server.

Network-attached storage consists of hard disk storage, including multi-disk RAID systems, and software for configuring and mapping file locations to the network-attached device.

Network-attached storage can be a step toward and included as part of a more sophisticated storage system known as a storage area network (SAN).

1.3 Storage Area Network (SAN):

A storage area network (SAN) is a type of local area network (LAN) designed to handle large data transfers. A SAN typically supports data storage, retrieval and replication on business networks using high-end servers, multiple disk arrays and Fibre Channel interconnection technology.

SAN technology is similar but distinct from network attached storage (NAS) technology. While SANs traditionally employ low-level network protocols for transferring disk blocks, a NAS device typically works over TCP/IP and can be integrated fairly easily into home computer networks.

A Storage Area Network (SAN) is a dedicated network that carries data between computer system and storage devices; which can include tape and disk resources. A SAN consists of a communication infrastructure which provides physical connections and a management layer which organizes the connection of storage elements and computer system so that the data transfer is secure and robust

The term SAN can sometimes refer to system area networks instead of a storage area network. System area networks are clusters of high performance computers used for distributed processing applications requiring fast local network performance.


2.1 DAS:

1962: IBM introduced the model 1301 Advanced Disk File. The key advance of this disk drive was the creation of heads that floated, or flew, above the surface of the disk on an "air bearing", reducing the distance from the heads to the surface of the disks from 800 to 250 micro inches.

1973: 1973, IBM introduced the model 3340 disk drive, which is commonly considered to be the father of the modern hard disk. This unit had two separate spindles, one permanent and the other removable, each with a capacity of 30 MB

2.2 NAS:

Historically, floppy drives have been widely used to share data files, but today the storage needs of the average person far exceed the capacity of floppies.

Businesses now maintain an increasingly large number of electronic documents and presentation sets including video clips. Home computer users, with the advent of MP3 music files and JPEG images scanned from photographs, likewise require greater and more convenient storage.

Network-attached storage was introduced with the early file sharing Novell's NetWare server operating system and NCP protocol in 1983

Starting in the early 2000s, a series of startups emerged offering alternative solutions to single filer solutions in the form of clustered NAS - Spinnaker Networks (now acquired by NetApp), Exanet, IBRIX, Isilon, PolyServe (acquired by Hewlett-Packard in 2007), to name a few.

2.3 SAN:

In the mid 1990s Fibre-channel initially gained popularity in the pre-press industry in which large image files were moved around from workstations to other types of specialized printing equipment.

The Fiber-channel directory moved over from our Sun directory, over to STORAGE search in 1998, and at one time we had 5 concurrent advertisers promoting FC host bus adapters until the market consolidated.


3.1 Disadvantage of DAS:

DAS has been referred to as "Islands of Information". Disadvantages of DAS include inability to 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.

3.2 Benefits of NAS:

Performance can be increased by NAS because the file serving is done by the NAS and not done by a server responsible for also doing other processing. The performance of NAS devices, though, depends heavily on the speed of and traffic on the network and on the amount of cache memory (RAM) on the NAS computers or devices.

It should be noted that NAS is effectively a server in itself, with all major components of a typical PC - a CPU, motherboard, RAM, etc. - and its reliability is a function of how well it is designed internally. A NAS without redundant data access paths, redundant controllers, redundant power supplies, is probably less reliable than Direct Attached Storage (DAS) connected to a server which does have redundancy for its major components.

3.3 Disadvantage of NAS:

Certain NAS devices fail to expose well-known services that are typical of a file server, or enable them in a way that is not efficient. Examples are: ability to compute disk usage of separate directories, ability to index files rapidly (locate), ability to mirror efficiently with rsync. One may still use rsync, but through an NFS or CIFS client; that method fails to enumerate huge file hierarchies at the nominal speed of local drives and induces considerable network traffic.

The key difference between DAS and NAS is that DAS is simply an extension to an existing server and is not networked while NAS sits on a network as its own entity; it is easier to share files with NAS. NAS typically has less CPU and I/O power compared to DAS.

3.4 Benefits of SAN:

Sharing storage usually simplifies storage administration and adds flexibility since cables and storage devices do not have to be physically moved to move storage from one server to another.

Other benefits include the ability to allow servers to boot from the SAN itself. This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server. This process can take as little as half an hour and is a relatively new idea being pioneered in newer data centers.

There are a number of emerging products designed to facilitate and speed up this process still further. For example, Brocade offers an Application Resource Manager product which automatically provisions servers to boot off a SAN, with typical-case load times measured in minutes. While this area of technology is still new, many view it as being the future of the enterprise datacenter.

SANs also tend to enable more effective disaster recovery processes. A SAN could span a distant location containing a secondary storage array.

This enables storage replication either implemented by disk array controllers, by server software, or by specialized SAN devices. Since IP WANs are often least costly method of long-distance transport, the Fibre Channel over IP (FCIP) and iSCSI protocols have been developed to allow SAN extension over IP networks.

The traditional physical SCSI layer could only support a few meters of distance - not nearly enough to ensure business continuance in a disaster. Demand for this SAN application has increased dramatically after the September 11th attacks in the United States, and increased regulatory requirements associated with Sarbanes-Oxley and similar legislation.

Consolidation of disk arrays economically accelerated advancement of some of their advanced features. Those include I/O caching, snapshotting and volume cloning (Business Continuance Volumes or BCVs).

A SAN uses fibre channel transport which is set of standards which define protocols for performing high speed serial data transfer, up to 400 Mbps. It provides a standard transport medium over which computer system communicate with devices such as disk storage arrays.

SCSI over Fibre channel implementation allow these devices to be connected in dynamic fibre channel topologies which span much greater distances and provide a greater level of flexibility and manageability while retaining the basic functionality of SCSI.

This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server. This process can take as little as half an hour and is a relatively new idea being pioneered in newer data centers.

As it is a networked infrastructure, many devices and host can be attached seamlessly, upwards of 16 million devices in a SAN. This allows better utilization of corporate assets and case of management both for configuration and security.

There are five components of SAN. They are:

  • Host Bus Adapter (HBA):
  • HBA's can be compared with to a NIC in LAN, as they provide a critical link between SAN and the operating system and the application software.

  • Connectivity:
  • To connect the nodes ,optical fiber cables are used. There are two types of cable employed in a SAN - Multimode and Single mode.

  • Connectors:
  • Optical and Electrical connectors are used in SANs.

  • Connectivity Devices:
  • For Fibre channel SANs, connectivity is provided by Fibre channel hubs and switches. These devices act as the common link between nodes within the SAN.

  • SAN Management Software:

SAN management software provides a single view of your storage environment. Management of the resources from one central console is simpler and more efficient.

5. Types of DAS:

5.1 SCSI:

SCSI is a standard that defines an interface between an initiator (usually computer) and a target (usually a storage device such as a hard disk).

Interface refers to connectors, cables, electrical signals, optical signals and the command protocol that allow initiators and targets to communicate.

5.2 ATA:

AT Attachment (ATA) and AT Attachment Packet Interface (ATAPI) are interface standards for the connection of storage devices such as hard disks, solid-state drives, and CD-ROM drives in computers.

The current ATA/ATAPI standard is the result of a long history of incremental technical development. ATA/ATAPI is an evolution of the AT Attachment Interface, which was itself evolved in several stages from Western Digital's original Integrated Drive Electronics interface. As a result, many near-synonyms for ATA/ATAPI and its previous incarnations exist, including abbreviations such as IDE which are still in common informal use. With the market introduction of Serial ATA in 2003, the original ATA was retroactively renamed Parallel ATA (PATA)

The first implementation of SAN was a simple grouping of hosts and associated storage in a single network often using a hub as the connectivity device. This configuration is called Fibre Channel Arbitrated Loop (FCAL).

As demand increased and technology improved, Fibre Channel switches greatly increased connectivity and performance allowing for interconnected SANs and ultimately enterprise level data accessability of SAN application and accessability

Fibre Channel is set of standards which define protocols for performing high speed serial data transfer. The standards define a layered model similar to OSI model found in traditional networking technology.

Fibre Channel has become widely used to provide a serial transport medium over which computer system communicate with devices such as disk storage arrays. The devices have traditionally been attached to system over more traditional channel technologies such as SCSI. SCSI over Fibre Channel implementations now allow these devices to be connected in dynamic Fibre Channel topologies which span much greater distances and provide a greater level of flexibility and manageability than found with SCSI. Fibre Channel networks are often reffered to a networks that perform channel operations.

6.2 IP SAN (Internet Protocol SAN):

IP technology is emerging as an alternative or supplemental transport for storage traffic. IP network has been at the file system level. Now emerging technologies provide for the transfer of block-level data over and existing IP network infrastructure.

IP is being positioned as a storage transport because IP management is easier. Existing network infrastructure can be leveraged, reducing the cost when compared with an investment in new SAN hardware and software. IP support multi-vendor interoperability. Many long-distance disaster recovery solutions already leverage IP-based networks. Many robust and mature security options are available for IP networks.


7.1 Architecture of DAS:

SANs are primarily used in large scale, high performance enterprise storage operations. It would be unusual to find a single disk drive connected directly to a SAN. Instead, SANs are normally networks of large disk arrays. SAN equipment is relatively expensive; therefore, Fibre Channel host bus adapters are rare in desktop computers. The iSCSI SAN technology is expected to eventually produce cheap SANs, but it is unlikely that this technology will be used outside the enterprise data center environment. Desktop clients are expected to continue using NAS protocols such as CIFS and NFS. The exception to this may be remote storage replication.

Video editing workgroups require very high data rates. Outside of the enterprise market, this is one area that greatly benefits from SANs.

Per-node bandwidth usage control, sometimes referred to as quality-of-service (QoS), is especially important in video workgroups as it lets you ensure a fair and prioritized bandwidth usage across your network. Avid Unity, Apple's Xsan and Tiger Technology MetaSAN are specifically designed for video networks and offer this functionality.


Storage Area Network can handle large amount of block level I/O and are suited to meet the demands of high performance applications that need access to data in real time.

In several, these applications have to share access to storage resources and implementing them in a SAN allow efficient use of these resources.

When data volatility is high, a host's need for capacity and performance can grow or shrink significantly in a short period of time. The SAN architecture is flexible, so existing storage can be rapidly redeployed across hosts, as need change with minimal disruption.

Hence, STORAGE AREA NETWORKWhere information lives...


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