Storage 101: What is Networked Storage?

The first requirement--sharing data between servers--is addressed by Network Attached Storage (NAS). A NAS device provides file access to clients that it connects to using file access protocols (primarily CIFS and NFS) transported on Ethernet and TCP/IP.

Storage Area Networks (SANs) allow multiple servers to share disk space from one or more disk arrays. SANs provide block-level storage access to servers using Fibre Channel technology.

In the NAS paradigm, the file system that organizes blocks of storage into objects that are convenient for applications to deal with resides in the storage device. The NAS storage device is responsible for allocating storage space and for keeping clients from stepping on each other's toes as they make file access requests. On the host side of the interconnect, a file access client translates applications' file I/O requests into network messages and sends them to the NAS device for execution.

By contrast, in today's SAN paradigm, the file system is on the computer side of the interconnect. System-wide storage capacity management and conflicts among client data access requests are resolved by cooperation among the SAN-attached servers. This makes host-side software much more complex than with NAS devices.

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By absorbing the file system into the storage device, the NAS model makes concurrent data access by different types of computers easy. In fact, today, NAS is the only widely available way to make the same data accessible to computers of different types.

Additionally, NAS file access protocols are very general and functionally rich. Moreover, they usually connect to TCP/IP-based networks, which are designed to support very general interconnection topologies. Because of their functional richness and generality, these protocols are predominantly implemented in software, which executes slowly compared to the device-level firmware and hardware typically used to implement SAN protocols. Raw data access performance of NAS device, therefore, tends to be lower than that of otherwise comparable SAN devices, and both client and server processor utilization for accessing data tends to be higher. In simple terms, the trade-off today is, therefore, as follows: