DISTRIBUTED FILE SYSTEMS

Presented By HariKrishnan

S7CSE

Computing System is a collection of processes operating on data objects. Persistent data objects should be named and saved on nonvolatile storage device.

Named data objects are files.

A file system is a major component in an OS.

A Distributed File System(DFS) is an implementation of file system.

Important concepts in distributed System design• DFSs employ many aspects of the notion of transparency.

• The directory service in DFS is a key component in all distributed systems.

• The performance and availability require the use of caching and replication.

• Access control and protection for DFSs open many problems in distributed system security.

Characteristics of DFS:dispersion and multiplicity of users and

files.Transparent DFS should exhibit the following properties:

Dispersed Clients

Dispersed Files

Multiplicity of users

Multiplicity of files

DFS DESIGN AND IMPLEMENTATIONBasic concepts of files and file systems: Files consists of three logical components.

File attributes

Data unitsFile name

File accesses are generally in one of the three modes: Sequential access, Direct access , indexed sequential access.

A file system consists of four major components:

I. DirectoryII. AuthorizationIII. File serviceIV. System services

Directory service Name resolution, add & deletion of files

Authorization service Capability and/or access control list

File service

Transaction

Concurrency & replication management

basic read./write files and get/set attributes

System services Device, cache and block mgmt

The organization of data files can be either flat or hierarchical. Files are named and accessed using a hierarchical pathname .

root

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reportbook Paper

Directories are files that contain names and addresses of other files and subdirectories.

File access must be regulated to ensure security.

Directory , Authorization and file services are user interfaces to a file system.

System services are file systems interface to hardware & are transparent to users.

Major Fns of System services includes:• mapping of logical to physical block addresses• Interfacing to services at the device level for file space allocation/deallocation• Actual read/write file operations.

Services and Servers: Servers are processes that implement services. A service may be implemented by a server/ number of servers. A server may also provide multiple services. Interaction among services in DFS:

Directory service

Authorization service

File service

System service

Serversclients

File mounting and Server Registration Constructs a large file system from various file servers and storage devices Mounting point is usually the leaf of directory tree that contain only an empty subdirectory Once files are mounted they’re accessed using the concatenated logical path names. File system mounting can be done in three diff instances:o Explicit mountingo Boot mountingo Auto mounting

Stateful and Stateless File ServersA connection requires the establishment and termination of communication session. There’s state information associated with each session.Ex: Opened files and their clients File descriptors and File handles Current file position pointers Mounting information Lock status Session keys Cache/Buffer

A file server is stateful if it maintains internally some of the state information and stateless if it maintains none at all.

Implementation of stateless server must address the following issues:

Idempotency Requirement File locking mechanism Session key management Cache consistency

File Access and Semantics of sharing

File sharing- multiple clients access same file at same time. The may result from either overlapping/interleaving

Coherency Control- Managing access to the replicas, to provide a coherent view of the shared file

Concurrency control- Concurrency is achieved by time multiplexing of the files and the issues here are how to prevent one execution sequence from interfering with others when they’re interleaved & how to avoid inconsistent results.

In space domain read and write accesses to a remote file can be implemented in one of the following ways: 1.Remote access 2.Cache access 3.Download/upload access Coherency of replicated data may be interpreted in many diff ways 1. All replicas are identical in all times2. Replicas are perceived as identical only at some points in time3. Users always read the “most recent”data in the replicas.4. Write operations are always performed immediately and their results are propagated in a best -effort fashion

In time domain interleaved read and write results in concurrent file accesses. 1. Simple RW 2. Transaction 3. Session

Semantics of sharing

Solutions to coherency and concurrency control problem depends on semantics of sharing.

Three popular semantic models:

• Unix semantics• Transaction semantics• Session semantics

Version control:All problems associated with file sharing and replication disappear if the file is read only. To achieve write sharing clients must know the names of newly created files.

A simple solution is to use same file name but with a version number for each revision of the file.

The burden of enforcing file sharing semantics is separated from the file service in to a higher level service called version control.

File with highest version number considered to be current version.

Thank You