 Hello and welcome to this session on file accessing models and file sharing semantics in distributed file system. At the end of this session, students will be able to uniquely identify the process of the file access models and file sharing semantics in distributed file system. In a computer system, a file is named object that comes into existence by explicit creation which is insusceptible to temporary failures in the system and pursuits until explicitly destroyed. The two main purposes of using files are permanent storage of information. This is achieved by storing a file on a secondary storage media such as magnetic disk. Second sharing of information files provides for natural and easy means of information sharing that is in the form of file can be created by one application and then shared with different application after some time. A file system is subsystem of an operating system that performs file management activities such as organization, storing, retrieval, naming, sharing and protection of files. It is designed to allow programs to use a set of operations that characterize the file abstraction and free the programmers from concerns about the details of space allocation and the layout of secondary storage device. Therefore, a file system provides an abstraction to a storage device which is convenient mechanism for storing and retrieving information from storage device. A distributed file system provides similar abstraction to the users of a distributed system and makes it convenient for them to use the files in a distributed environment. The design and implementation of a distributed file system, however, is more complex than conventional file system due to the fact or the reason that the users and the storage devices are physically distributed. Despite of the advantages of permanent storage and sharing of information provided by the file system in a single processor system, distributed file systems normally supports remote information sharing. A distributed file system allows a files to be transparently accessed by processes of any nodes of the system irrespective of the file location. Therefore, a process on one node can create a file that can be accessed at later time by some other process running on another node. User mobility in a distributed system, user mobility implies that a user should not be forced to work on a specific node but should have the flexibility to work on different nodes at different times. In distributed file system, normally allows a user to work on different nodes at different times without the necessity of physically relocating the secondary storage devices. Availability, for better fault tolerance, files should be available for use even in the event of the temporary failure of one and more nodes of the system. To take care of this, a distributed file system normally keeps multiple copies of a file on different nodes on the system. Each copy is called replica of the file. Dyslex Workstations Disk drives are relatively expensive compared to the cost of most of the other parts in a workstation. However, workstations is likely to be physically placed in the intermediate vicinity of the user. That's why workstations is more economical and is less noisy and generates less heat. The good distributed file system should have the features first transparency. There are four types of transparencies, structure transparency. For performance scalability and reliability reason, a distributed file system normally uses multiple file servers. Each file server is normally used, user process and kernel process that is responsible for controlling a set of secondary storage devices of the node on which it runs. In multiple file servers, the multiplicity of the file server should be transferred to the clients of a distributed file system. In access transparency, both local and remote files should be accessible in the same way. The file system interface should not distinguish between the local and the remote files. And the processing should automatically locate and access file and arrange for the transport of data to the client side. Naming transparency. The naming of the file should give no hint as to where the file is located. A file should be allowed to move from one node to another in a distributed system without having to change the name of the file. Replication transparency. If a file is replicated on multiple nodes, both the existence of multiple copies and their location should be hidden from the clients. User mobility. In a distributed system, users should not be forced to work on specific nodes but should have the flexibility to work on different nodes at different times. Performance. The performance of a file system is usually measured as the average amount of time needed to satisfy client request. In a distributed file system, however, this time also includes the network communication overhead when the access file is remote. Simplicity and ease of use. Several issues influences the simplicity and the ease of use in the distributed file system. The most important issue is the semantics of distributed file system should be easy to understand. Scalability. Distributed system will grow with the time since expanding the network by adding new machines or interconnecting two networks together. Therefore, a good distributed file system should be designed easily to cope up with the growth of nodes and users in the system. High availability. Distributed file system should continue to the function even when partial failures due to the one or more components such as communication link failure, a machine failure or a storage device crash. When a partial failure occurs, the system may show degradation in performance or the functionality or both. High availability and scalability are mutually related properties. High reliability. In a good distributed file system, the probability of loss of stored data should be minimized. Stable storage is a popular technique used by several file systems for high reliability. Data integrity. A file is often shared by multiple users for a shared file. The file system must guarantee the integrity of data stored in it. The concurrent access request from multiple users who are competing to access the file must be properly synchronized by the use of some form of concurrency control mechanism. Security. Distributed file system should be secure so that its user can be confident of the privacy of the data. Necessary security mechanism must be implemented to protect information stored in a file system against unauthorized access. Hydrogenity. As a consequence of large scale, hydrogenity becomes inevitable in a distributed system. Hydrogenous distributed system provides the flexibility to the user to use different computer platforms for different applications. Distributed file system may use one of the models shown here to service client's file access request when the access file is a remote file. Remote service model. In this model, the processing of the client request is performed at the service node. That is, the client's request for the file access is delivered to the server service machine, performs accept request and finally the result is forwarded back to the client. The access request from the client and the server replies for the client or transfers across the network as messages. Here the diagram shows the client server model, the request from client to access remote files and the file server will respond the request coming from the client. Data caching model. This model attempts to reduce the amount of network traffic by taking advantage of locality features founded in the file access. In this model, the data needed to satisfy the client access request is not present locally. It is copied from the server's node to the client node and is captured there. The client request is processed on the client nodes itself by using the cache data. The recently accessed data is retained in the cache for some time so that repeated access of the same data can be handled locally. The diagram shows the model for data caching. Here pause the video, think and answer. In a distributed file system, when a file physical location changes, the file need not to be changed. The answer is B. Here are my references. Thank you.