DISTRIBUTED FILE SYSTEMS
A file system is responsible for the organization, storage, retrieval, naming, sharing, and protection of files. File systems provide directory services, which convert a file name (possibly a hierarchical one) into an internal identifier (e.g. inode, FAT index). They contain a representation of the file data itself and methods for accessing it (read/write). The file system is responsible for controlling access to the data and for performing low-level operations such as buffering frequently used data and issuing disk I/O requests.
A distributed file system is to present certain degrees of transparency to the user and the system: Access transparency: Clients are unaware that files are distributed and can access them in the same way as local files are accessed.
Location transparency: A consistent name space exists encompassing local as well as remote files. The name of a file does not give it location.
Concurrency transparency: All clients have the same view of the state of the file system. This means that if one process is modifying a file, any other processes on the same system or remote systems that are accessing the files will see the modifications in a coherent manner.
Failure transparency: The client and client programs should operate correctly after a server failure.
Heterogeneity: File service should be provided across different hardware and operating system platforms.
Scalability: The file system should work well in small environments (1 machine, a dozen machines) and also scale gracefully to huge ones (hundreds through tens of thousands of systems).
Replication transparency: To support scalability, we may wish to replicate files across multiple servers. Clients should be unaware of this.
Migration transparency: Files should be able to move around without the client's knowledge. Support fine-grained distribution of data: To optimize performance, we may wish to locate individual objects near the processes that use them.
Tolerance for network partitioning: The entire network or certain segments of it may be unavailable to a client during certain periods (e.g. disconnected operation of a laptop). The file system should be tolerant of this.
File service types
To provide a remote system with file service, we will have to select one of two models of operation. One of these is the upload/download model. In this model, there are two fundamental operations: read file transfers an entire file from the server to the requesting client, and write file copies the file back to the server. It is a simple model and efficient in that it provides local access to the file when it is being used. Three problems are evident. It can be wasteful if the client needs access to only a small amount of the file data. It can be problematic if the client doesn't have enough space to cache the entire file. Finally, what happens if others need to modify the same file?
The second model is a remote access model. The file service provides remote operations such as open, close, read bytes, write bytes, get attributes, etc. The file system itself runs on servers. The drawback in this approach is the servers are accessed for the duration of file access rather than once to download the file and again to upload it.
Another important distinction in providing file service is that of understanding the difference between directory service and file service. A directory service, in the context of file systems, maps human-friendly textual names for files to their internal locations, which can be used by the file service. The file service itself provides the file interface (this is mentioned above). Another component of file distributed file systems is the client module. This is the client-side interface for file and directory service. It provides a local file system interface to client software (for example, the vnode file system layer of a UNIX kernel).
· File system were originally developed for centralized computer systems and desktop computers.
· File system was as an operating system facility providing a convenient programming interface to disk storage.
· Distributed file systems support the sharing of information in the form of files and hardware resources.
· With the advent of distributed object systems (CORBA, Java) and the web, the picture has become more complex.
· Figure 1 provides an overview of types of storage system.
Figure 1. Storage systems and their properties
· Figure 2 shows a typical layered module structure for the implementation of a non-distributed file system in a conventional operating system.
· Figure 2. File system modules
· File systems are responsible for the organization, storage, retrieval, naming, sharing and protection of files.
· Files contain both data and attributes.
· A typical attribute record structure is illustrated in Figure 3.
Figure 3. File attribute record structure
· Figure 4 summarizes the main operations on files that are available to applications in UNIX systems.
Distributed File system requirements
Related requirements in distributed file systems are:
· Fault tolerance
File service architecture • This is an abstract architectural model that underpins both NFS and AFS. It is based upon a division of responsibilities between three modules – a client module that emulates a conventional file system interface for application programs, and server modules, that perform operations for clients on directories and on files. The architecture is designed to enable a stateless implementation of the server module.
SUN NFS • Sun Microsystems’s Network File System (NFS) has been widely adopted in industry and in academic environments since its introduction in 1985. The design and development of NFS were undertaken by staff at Sun Microsystems in 1984. Although several distributed file services had already been developed and used in universities and research laboratories, NFS was the first file service that was designed as a product. The design and implementation of NFS have achieved success both technically and commercially.
Andrew File System • Andrew is a distributed computing environment developed at Carnegie Mellon University (CMU) for use as a campus computing and information system. The design of the Andrew File System (henceforth abbreviated AFS) reflects an intention to support information sharing on a large scale by minimizing client-server communication. This is achieved by transferring whole files between server and client computers and caching them at clients until the server receives a more up-to-date version.
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