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1. DAS - Direct Access Storage

DAS (Directed Attached Storage) is the most common and basic form of storage networking. It generally comprises of storage devices that reside within network server or are connected directly to it through a separate cable using specialized bus adapter. The storage medium could be hard disks or tape drives and the most common type of protocols used in DAS are Small Computers System Interface (SCSI), Fiber Channel and Serial Storage Architecture (SSA).

DAS is easy to install and manage when compared to other storage technologies. It is also one of the cheaper storage options if not the cheapest. Other benefits of DAS include good performance and flexibility, that is, it can be attached to servers on a multitude of platforms. While DAS provide those benefits, DAS systems are not scalable enough, and can connect to one server only. If that particular server is down, the storage is not accessible to users. This also results in under-utilization of resources since the storage capacity available to one server may not be available to the other server. These limitations, and a bid to overcome them, led to the development of NAS.


2. NAS - Network Attached Storage

NAS unlike DAS is more of a device and less of a part of the network infrastructure. Typically, a NAS device may consist of a server and a stripped down version of an operating system optimized for storage; and a storage that is accessible and shared across the network. NAS devices are like the Plug-and-Play hardware in your machine. Just plug them into your existing LAN and they're ready to store. They support all major network protocols and, hence, are absolutely apt for heterogeneous networks. Unlike DAS and SAN these devices use file I/O instead of block I/O. The NAS server receives the request as a file to be accessed and its offset rather than an address to the storage device. The OS on the NAS servers carries out this function. 

NAS provides various benefits including ease of implementation and the fact that it allows organizations to exploit their existing network for storage purposes. Other benefits are greater scalability that can be achieved by adding NAS devices when needed, easier backup options that can be built in into the NAS server and easy manageability. The flip side, however, is that the networks on which NAS is used are not optimized for storage, and usage of the same network for storage may cause serious network congestion. 


3. SAN - Storage Area Network

SAN is a dedicated, high-speed network behind the servers for storage. Various servers and storage devices may attach to SAN making it an any-to-any network. A SAN generally works on a Fibre channel or SCSI network. SAN allows heterogeneous servers to share a pool of storage devices that can either be in the same vicinity or kilometers apart. Since SAN is a separate network in itself, it does not affect network traffic as much as NAS does- this makes the traffic really fast. Other benefits of SAN may include easier and improved backup and recovery options, and even easier scalability than NAS. SAN can be implemented using two topologies over the Fibre Channel network: Arbitrated Loop and Switched Fabric.

The biggest hindrance in SAN implementation is the cost - of the equipment involved, Fibre Channel switches, hubs etc. as well as the costs of maintaining virtually another network. The latest in the list of standards that have been discussed above is iSCSI (Internet Small Computer Systems Interface). It became an official standard in February 2003. This claims to eradicate the problems faced by SAN and Fibre Channel implementations, namely, cost and access to remote devices. iSCSI is an IP-based protocol that encapsulates SCSI commands in TCP/IP packets and enables I/O block data transport over IP networks. iSCSI aims at bringing the functionality of SAN to the current LAN networks to provide a high degree of interoperability.

4. iSCSI Storage Area Network

iSCSI allows for block-level storage over common IP networks. The appeal of iSCSI comes from its ability to take advantage of the existing IP network infrastructure and IP skill sets. From switches to adapter cards to even the storage devices themselves, IP storage will save the organization money. Early adopters report saving 30%, 50% or more over the cost of the equivalent amount of FC storage. SMEs will find iSCSI especially attractive for this reason.

As the foundation for IP-based storage, iSCSI creates many opportunities. In addition to enabling companies to create affordable SANs using their existing IP infrastructure and skills, it allows companies to leverage existing TCP/IP network links to move large volumes of block-level stored data over long distances. TCP/IP lowers the cost of asynchronous mirroring and replication, putting it in reach of more companies and making it feasible for more uses.

Companies can use IP-based remote snapshot/replication to maintain concurrent or nearly concurrent data at remote sites for purposes of balancing workloads between multiple data centers, application development and testing or business continuity and disaster recovery. iSCSI isn't perfect, especially at this still-early stage in its evolution. For example, users must contend with considerable IP overhead. The solution is to move the IP processing into a TCP/IP offload engine (TOE), which can be built as a chip and integrated into iSCSI cards and devices. Expect more chip-level TOEs to come in 2004.