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CSE 8343 Adv. OS Group A2 Distributed File SystemOutlineTerminologyTerminology (Cont’d)Overview of Centralized File SystemCont’dDistributed File SystemSignificant Features of DFSStorage ManagementDFS DesignFile Service Interface (cont’d)2- Directory service InterfaceNaming in DFSNaming (Cont’d)Semantics of File SharingSemantics of File Sharing Issues of File SharingSlide 17Slide 18Semantics of file SharingDFS :CachingDFS: CachingSlide 22Client Cache Location IssuesSlide 24Slide 25Cache ConsistencySlide 27Stateful File ServiceStateless File ServerStateful Vs StatelessStateful Vs Stateless cont..File ReplicationFile Replication Cont…Network File System (NFS)NFS DesignNFS ArchitectureNFS Architecture cont.Benefits of NFS ?Problems with NFSAlternatives to NFSWhy is NFS used then ?NFS Specifications.Mount ProtocolNFS ProtocolCommon NFS Errors:RPC ProtocolHow RPC works ?How RPC works cont.Any Questions ?01/14/19 1 CSE 8343 Adv. OS Group A2 Distributed File System by Sajida Begum Samina Choudhry01/14/19 2 OutlineTerminologyOverview of Centralized File SystemWhat is Distributed File System?Naming in DFSSemantics of DFSCachingStateful vs. stateless serviceFile ReplicationExamples01/14/19 3 Terminology Tightly coupled system: -Processors share memory and a clock. - Communication through shared memory. Loosely Coupled System: - No shared memory or clock - Each process has its own local memory. - Communication through various communication lines e.g.high speed buses and telephone lines. Distributed System: It is a collection of loosely coupled machines inter connected by communication network.01/14/19 4 Terminology (Cont’d) Remote and Local Resources: In a distributed system, the resources of a specific processor are called local while the rest of the processors and their respective resources are remote. Service: A software entity running on one or more machines and providing a particular type of function to a priori unknown clients. Server: The service software running on a Single machine. Client: A process that can invoke a service using a set of operations that forms its client interface.01/14/19 5File Naming: c:\courses\cse8343\proc.ps (MS-DOS) /usr/schoudhr/courses/cse8343/process.ps(UNIX)File Structure:Bitstream or bytestreamRecord oriented (record = key + data)Indexed(e.g, B*-trees (IBM VSAM))File Types:Text (e.g., ASCII)Binary (e.g.,executables, images, etc.)Overview of Centralized File System01/14/19 6Cont’d Directory Structures- Flat- hierarchical(tree)-graphAllocation of File to Disk Blocks- contiguous- linked- indexed- i-node(UNIX)01/14/19 7Distributed File System Def: DFS is a distributed implementation of the classical time sharing model of a file system, where multiple users share files and storage resources. OR A DFS is a file system whose clients, servers and storage devices are dispersed among the machines of a distributed system. Object: The purpose of a DFS is to support the same kind of sharing when the files are physically dispersed among the various sites of a distributed system.01/14/19 8Significant Features of DFS-No single centralized data repository-Multiple and independent storage devices-Multiplicity of clients & servers The multiplicity and dispersion of servers and storage devices should be made transparent i.e. the client interface of a DFS should not distinguish between local and remote files. - Autonomy of Clients and Servers - Transparent DFS It facilitates user mobility by bringing over the user’s environment(home directory) to wherever a user logs in.01/14/19 9Storage Management- DFS manages a set of dispersed storage devices. The overall storage space managed by a DFS is composed of different, and remotely located , smaller storage spaces.- Correspondence between storage spaces and-Set of files.- All files belonging to the same component unit must reside in the same location.-Where component unit is the smallest set of files that can be stored on a single machine , independently from other units.01/14/19 10 DFS DesignA DFS typically has two reasonably distinct components:1-File Service Interface:Upload/download model 1- file moved to a client old file  new file client step:3 server step:2 1- File moved to client2- entire files are retrieved from the server, and accessed at the client.3-once the client is done, the file is stored back at the server.-typical of mass storage system: e.g. Unitree.01/14/19 11File Service Interface (cont’d)-remote access model Client Server Requests from client to access remote file file stays on server - only the needed blocks of files are retrieved from- the server. - Once the client is done with a block, it is written back to the server. - Example : NFS01/14/19 122- Directory service InterfaceFile server 1 Client 1 client 1  B A  File server 2 Client 2 Client 2 a b cAB CB CBDArootDAC FEDFErootEFrootrootDAACB E B CDEEF01/14/19 13Naming in DFS Def:Naming is a mapping between logical and physical objects. Naming Structure: Location Transparency: The name of a file does not reveal the file location. e.g.:/serverA/dir1/dir2/x does not show where the server is located. Location Independence: Files can be moved and all references to them continue to be valid i.e. the name of a file does not has to be changed with any change in file’s physical storage location.01/14/19 14 Naming (Cont’d)Two Level Naming- Symbolic Names: human readable.  e.g.:/courses/slides/files.ps- Binary Names: machine readable names.Easier to manipulate.e.g.:UNIX i-node , or server IP address: i-node number- Symbolic to binary name mapping may be one to many in a distributed system (file replication)01/14/19 15Semantics of File SharingUnix Semantics: used in centralized systems. - a read that follows a write sees the value written by the write. write x’ to block


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