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Network Attached Storage (NAS) represents a more recent advance in network storage, allowing devices to be connected directly to the network rather than to a server system in DAS. NAS devices are typically self-contained, with an embedded OS providing the functionality to clients. NAS devices are assigned an IP address which clients can then use to access the NAS device directly, or through a server which acts as a gateway.
While NAS provides a high level of flexibility and availability, it only operates as well as the network and IP allow. Some applications that transfer large blocks of data can slow down the network, and there remains a single path in and out of the disk subsystem. Unlike DAS, however, NAS can be located anywhere on the network. In addition, data can be placed on separate NAS drives and placed close to the users who use it as a way to avoid traffic overflows. Finally, NAS allows multiple operating platforms to access a single drive.
Although there are distinct advantages for NAS over DAS, particularly in terms of flexibility, there are additional concerns. If data is spread across multiple NAS drives, managers need to ensure that the information remains synchronized and reliable. In addition, as is true with most IP-based services, security becomes a concern. Posting storage medium on a network potentially exposes that data to anyone who can access the network, and so requires an additional level of data encryption and security.
Storage Area Networks (SANs) are highly touted as the solution to increasing demands for reliable and flexible network storage solutions. A SAN is effectively a mini network created between storage devices that allow the devices to communicate with each other without using of the standard network infrastructure. In addition, SAN's make it possible for multiple server system to access the same storage devices allowing the centralization of data.
In a SAN, disks attach to bridges and controllers that create dynamic links between the data disk and the server or workstation. Generally called Fibre Channel subsystems, the physical connections can be copper or fiber optic wiring.
The use of a bridge or switch to connect the SANs offers several benefits. In essence, any device can attach to a disk and operate as though it were a private network. This lessens the impact that heavy transactions and backup activities can have on network performance. Data transmission rates are 100MB to 200MB per second, and most expect these speeds to reach 2GB per second. In addition SAN's can be easily expanded.
Fibre Channel implements a layered architecture that control protocols and protocol translation. Fibre Channel SAN standards exist, and most fibre channel vendors claim to want open systems. However, interoperability remains elusive. The current specification focuses on transporting data and the protocols associated with that function. As a result, each vendor can (and does) implement a different management and control system. Further, vendors frequently extend their products, and this can prevent customers from mixing products from separate vendors on the same fibre channel.
Direct Attached Storage (DAS)
Network Attached Storage (NAS)
Storage Area Network (SAN)
What Runs With What?
In response to concerns over product and technology compatibility, several leading vendors support and participate in the accreditation programs discussed earlier. Under these programs, vendors can submit products for testing to determine compatibility with the other vendor's products.
Competing vendors also develop proprietary standards that compete directly with the "standard" technologies. IBM, for example, developed SSA, and several manufacturers offer products for that specification. IT managers face a difficult prospect of studying the competing standards and separating fiction from fact. In addition, new technologies will enter the market to challenge established products.
No wonder, IT managers scratch their heads.