Distributed File Systems
Dr. Wajeeha Khalil
MS(CS) Fall 2013
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Distributed File Systems: Sharing and Storing Information in a Networked Environment, Thesis of Computer Networks
An introduction to distributed file systems (dfs), their goals, advantages, and requirements. Dfs enables remote information sharing, user mobility, high availability, and diskless workstations. It consists of three main services: storage service, true file service, and name service. This resource covers desirable features, file models, and accessing remote files, using examples like sun network file system and nfs.
Typology: Thesis
2015/2016
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Distributed File Systems
Dr. Wajeeha Khalil MS(CS) Fall 2013
Introduction
- (^) File: named object that comes into existence by explicit creation
- (^) Immune to temporary failure in the system
- (^) Persists until explicitly destroyed
- (^) Two main served purposes
- (^) Permanent Storage of Information
- (^) Storing a file on secondary storage device
- (^) Sharing of Information
- (^) File created by one application and shared with different applications
- (^) Permanent Storage of Information
File System Vs Distributed File
System
- (^) Subsystem of OS that performs file management activities - (^) Organization, Storing, Retrieval, Naming, Sharing and Protection - (^) Allows programs to use a set of operations that characterize the file abstraction and free the programmers from concerns about - (^) Space allocation, and Layout of the secondary storage device
- (^) Distributed File System
- (^) Provides Similar Abstraction to the users
- (^) More Complex than Conventional File System
Advantages of DFS
- (^) Remote Information Sharing
- (^) User Mobility
- (^) Availability
- (^) Diskless Workstations
Desirable Features of a Good
DFS
- (^) Transparency
- (^) Structure
- (^) Access
- (^) Naming
- (^) Replication
- (^) User Mobility
- (^) Performance
- (^) Simplicity and Ease of Use
- (^) Scalability
- (^) High Availability
Desirable Features of a Good
DFS
- (^) High Reliability
- (^) Data Integrity
- (^) Security
- (^) Heterogeneity
Distributed File System Requirements
- (^) Concurrent File Updates
- (^) Changes to a file by one client should not interfere with the operation of other clients simultaneously accessing or changing the same file - (^) Concurrency control - (^) File locking
Distributed File System Requirements
- (^) File Replication
- (^) A file may be represented by several copies of its contents at different locations - (^) Sharing of load - (^) Fault tolerance
- (^) Caching is implemented by some DFS (partial replication)
Distributed File System Requirements
- (^) Consistency
- (^) Non-DFS offer one-copy semantics
- (^) For replicated files, delay in update propagation - (^) May result in deviation from one-copy semantics
- (^) Strict consistency must be achieved
- (^) Efficiency
- (^) DFS should provide a service that is comparable with, or better than, local file system in performance
- (^) Convenient to administer
File Models
- (^) Different Conceptual Models
- (^) Two most commonly used criteria for file modeling are - (^) Structure - (^) Modifiability
- (^) Unstructured and Structured
- (^) Unstructured
- (^) No substructure known to the file server
- (^) Contents of each filer appear to the file server as an uninterrupted sequence of bytes
- (^) Meaning and Structure of the Data stored in files are entirely up to the application Programs
- (^) UNIX and MS-DOS use this file Model
- (^) Unstructured
File Models
- (^) Immutable
- (^) Example: Cedar File System
- (^) A file cannot be modified once it has been created except to be deleted.
- (^) File Versioning approach is normally used to implement the file updates.
- (^) Each file is represented by a history of immutable versions
File- Accessing Models
- (^) Manner in which a Client’s request to access a files is serviced depends on the file accessing model used by a FS
- (^) Accessing Remote Files
- (^) Unit of Data Transfer
Distributed File Systems
- (^) Support the sharing of information in the
form of files throughout an intranet
- (^) Performance and reliability similar to files stored on local disks
- (^) Sun Network File System, NFS
- (^) Andrew File System, AFS
- (^) Coda [TS]
- (^) xFS [TS]
- (^) SFS [TS]
File Service
- (^) A file service enables programs to store and access remote files exactly as they do local ones - (^) Persistent storage at a few servers - (^) Reduces the need for local disk storage - (^) Management and archiving facilities
- (^) Web servers are reliant on filing systems