Network Attached Storage

File Sharing Environment : File sharing, enables users to share files with other users . A user who creates the file (the creator or owner of a file) determines the type of access (such as read, write, execute, append, delete) to be given to other users. When multiple users try to access a shared file at the same time, a locking scheme is required to maintain data integrity and, at the same time, make this sharing possible.

Some examples of file-sharing methods are; File Transfer Protocol (FTP), Distributed File System (DFS), client-server models that use file-sharing protocols such as Network File System NFS and Common Internet File System CIFS, and the peer-to-peer (P2P) model.

NAS is a dedicated, high-performance file sharing and storage device. NAS enables its clients to share files over an IP network. NAS uses network and file-sharing protocols to provide access to the file data. These protocols include TCP/IP for data transfer, and Common Internet File System (CIFS) and Network File System (NFS) for network file service.Many NAS systems also support Hypertext Transfer Protocol (HTTP). NAS enables both UNIX and Microsoft Windows users to share the same data seamlessly.

A NAS device uses its own operating system and integrated hardware and software components to meet specific file-service needs. Its operating system is optimized for file I/O and, therefore, performs file I/O better than a general-purpose server

Components of NAS : A NAS device has two key components: NAS head and storage. In some NAS implementations, the storage could be external to the NAS device and shared with other hosts.

Common Internet File System (CIFS) : is a client-server application protocol that enables client programs to make requests for files and services on remote computers over TCP/IP. It is a public, or open, variation of Server Message Block (SMB) protocol.

It uses file and record locking to prevent users from overwriting the work of another user on a file or a record. It is Stateful protocol CIFS server maintains connection information regarding every connected client.

Users refer to remote file systems with an easy-to-use file-naming scheme:

\\server\share or \\servername.domain.suffix\share.

Network File System (NFS) is a client-server protocol for file sharing that is commonly used on UNIX systems. It uses Remote Procedure Call (RPC) as a method of inter-process communication between two computers.

NFS (NFSv3 and NFSv2) is a stateless protocol and uses UDP as transport layer protocol (or TCP for NSFv3). NFS version 4 (NFSv4): Uses TCP and is based on a statefulprotocol design. It offers enhanced security.

NAS I/O Operation: The NAS operating system keeps track of the location of files on the disk volume and converts client file I/O into block-level I/O to retrieve data and converts back to file I/O for client.

NAS Implementation – common NAS implementations are unified, gateway, and scale-out.

The unified NAS consolidates NAS-based (file-level) and SAN-based (block-level) data access within a unified storage platform. It supports both CIFS and NFS protocols for file access and iSCSIand FC protocols for block level access. A unified NAS contains one or more NAS heads and storage in a single system. The storage may consist of different drive types, such as SAS, ATA, FC, and flash drives, to meet different workload requirements.

Gateway NAS : device consists of one or more NAS heads and uses external and independently managed storage. Management functions in this type of solution are more complex than those in a unified NAS environment because there are separate administrative tasks for the NAS head and the storage. A gateway solution can use the FC infrastructure, such as switches and directors for accessing SAN-attached storage arrays or direct-attached storage arrays.

The gateway NAS is more scalable compared to unified NAS because NAS heads and storage arrays can be independently scaled up when required. NAS gateway and the storage system in a gateway solution is achieved through a traditional FC SAN.

Scale-out NAS implementation:  Pools multiple nodes together in a cluster. A node may consist of either the NAS head or storage or both. The cluster performs the NAS operation as a single entity. A scale-out NAS provides the capability to scale its resources by simply adding nodes to a clustered NAS architecture. The cluster works as a single NAS device and is managed centrally.

Scale-out NAS creates a single file system that runs on all nodes in the cluster. All information is shared among nodes, so the entire file system is accessible by clients connecting to any node in the cluster. Scale-out NAS stripes data across all nodes in a cluster along with mirror or parity protection. Scale-out NAS clusters use separate internal and external networks for back-end and front-end connectivity, respectively. An internal network (uses high-speed networking technology, such as InfiniBand or Gigabit Ethernet.) provides connections for intracluster communication, and an external network connection enables clients to access and share file data.

File-level Virtualization : File-level virtualization eliminates the dependencies between the data accessed at the file level and the location where the files are physically stored. It creates a logical pool of storage, enabling users to use a logical path, rather than a physical path, to access files. A global namespace is used to map the logical path of a file to the physical path names. File-level virtualization enables the movement of files across NAS devices, even if the files are being accessed

Examples

EMC Isilon : Scale-out NAS solution. Isilon has a specialized operating system called OneFS that enables the scale-out NAS architecture.

EMC VNX Gateway: Gateway NAS Solution: The VNX Series Gateway contains one or more NAS heads, called X-Blades, that access external storage arrays, such as Symmetrix and block-based VNX via SAN. The VNX Gateway supports both pNFS and EMC patented Multi-Path File System (MPFS) protocols.

References and further reading:
http://www.emc.com/corporate/glossary/network-attached-storage.htm
http://www.snia.org/sites/default/education/tutorials/2009/fall/networking/WolfgangSinger_NAS_ISCSI_Technology_Overview.pdf