Naming Schemes of Distributed file systems

Large networks often use a systematic naming scheme, such as using a location (e.g. a department) plus a purpose to generate a name for a computer. However, smaller networks will frequently use a more personalized naming scheme to keep track of the many hosts. Network naming can be hierarchical in nature, such as the Internet's Domain Name System. Indeed, the Internet employs several universally applicable naming methods: Uniform Resource Names (URN), Uniform Resource Locators (URL), and Uniform Resource Identifiers (URI).
A good naming scheme is scalable, unique, and easy to remember. The purpose of these naming schemes is to name network computers, but it can also be used to name projects, variables, streets, pets, kids, or any other project where unique names and remember able names are required.

There are three main approaches to naming files:
 
- Files are named with a combination of host and local name.
* This guarantees a unique name.
* It is neither location transparent nor location independent.
* Same naming works on local and remote files.
- Remote directories are mounted to local directories.
* A local system seems to have a coherent directory structure.  
* The remote directories must be explicitly mounted. The files are location independent.
* SUN (Network File System)NFS is a good example of this technique.
- A single global name structure spans all the files in the system.
* The DFS is built in the same way as a local file system.
* It is location independent.

Difference between Location independence and Static location transparency

There are few aspects that differentiates location independence and static location transparency :
- In location independence, divorcing data from location provides better abstraction fro files. Location independent files can be viewed as logical data containers that are not attached to specific storage location.
In static location transparency, the file name still denotes a specific, although hidden, set of physical disk blocks.

- In static location transparency, users can share remote files by simple naming the files in a location transparent manner, as though the files are local but nevertheless, logical names are still attached to physical storage devices.
Location independence promotes sharing the storage space as well as the data objects.
A possible benefit of such a view is the ability to balance the utilization of disks across the system.

- Location independence separates the naming hierarchy from the storage devices hierarchy and from the inter-computer structure.
In static location transparency, the correspondence between component units machines can be easily exposed.

Once the separation of name and location has been completed, files residing on remote server systems may be accessed by different clients. In fact, these clients may be diskless and rely on servers to provide all files, including the operating system kernel.

Naming and Transparency in Distributed File Systems

Naming is the mapping between logical and physical objects. In a conventional file system, it's understood where the file actually resides; the system and disk are known. In a transparent DFS, the location of a file, somewhere in the network, is hidden. File replication means multiple copies of a file; mapping returns a SET of locations for the replicas.

- Location transparency -
* The name of a file does not reveal any hint of the file's physical storage location.
* File name still denotes a specific, although hidden, set of physical disk blocks.
* This is a convenient way to share data.
* It can expose correspondence between component units and machines.

- Location independence :
* The name of a file doesn't need to be changed when the file's physical storage location changes.Dynamic, one-to-many mapping.
* Better file abstraction.
* Promotes sharing the storage space itself.
* Separates the naming hierarchy from the storage devices hierarchy.

A location independent naming scheme is a dynamic mapping, since it can map the same file name to different locations at two different times. Therefore, location independence is a stronger property than location transparency.
Most DFSs today support location transparent systems. They do not support migration and files are permanently associated with specific disk blocks.

Overview of Distributed File Systems (DFS)

A distributed file system or network file system is any file system that allows access to files from multiple hosts sharing via a computer network. This makes it possible for multiple users on multiple machines to share files and storage resources.
In order to understand the structure of a distributed file system, the terms service, server and client should be defined. A service is a software entity running on one or more machines and providing a particular type of function. A server is the service software running on a single machine. A client is a process that can invoke a service using a set of operations that forms its client interface.
A distributed file system (DFS) is a file system whose clients, servers, and storage devices are dispersed among the machines of a distributed system. A service activity has to be carried out across the network, and instead of a single centralized data repository, there are multiple and independent storage devices. the distinctive features of a DFS are the multiplicity and autonomy of clients and servers in the system.
A DFS should look to its clients like a conventional, centralized file system. The client interface of a DFS should not distinguish between local and remote files. The most important performance measurement of a DFS is the amount of time needed to satisfy various service requests. In a DFS, a remote access has the additional overhead attributed to the distributed structure. This overhead includes the time needed to deliver the request to the server, as well as the time for getting the response across the network back to the client. DFS manages a set of dispersed storage devices which is the DFS's key distinguishing feature.