The two basic forms of storage for host computers are direct-attached storage (DAS) and networked storage. DAS is storage that is directly attached to a host computer and is generally privately owned by that computer. Networked storage is storage that is connected to a host computer via some sort of network, such as an Ethernet network or Fibre Channel network, and can take several forms, including variations of Storage Area Networks (SAN) and Network Attached Storage (NAS).
There are two basic forms of networked storage: the Storage Area Network (SAN) and Network-Attached Storage (NAS) and they are generally distinguished by their Input/Output (I/O) characteristics. SAN is generally used for applications that require “block” I/O access. NAS is generally used for applications that require “file” I/O access. An application that uses block I/O is any application that reads or writes its data blocks directly to the storage device or subsystem, such as a databases, email servers, or file systems themselves (such as NTFS). SAN storage devices appear to the applications the same way that DAS devices do, allowing applications to use what appear to be local storage devices. An application that uses file I/O is one that makes its read and write requests in the form of files, such as a network client reading and writing files from a file server. NAS devices typically appear as one or more network file shares to the applications and users. NAS devices are actually host servers themselves that internally use a DAS or SAN I/O connection, but share a “file-system” type of view of their storage resources to other hosts on the network.
The primary reasons for using any form of networked storage are to overcome the disadvantages of the DAS storage model. The various implementations of networked storage can allow the storage to be located potentially many miles from the host CPU requiring the storage and can scale to hundreds, thousands or even millions of storage devices. In addition, the networked storage model allows storage to be placed into a “pool” that is not necessarily owned by any one application client or server but can be shared among many applications or servers.
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