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Chapter 19 - Distributed file systems. This chapter discusses different methods of organizing access to files and directories located in various nodes of a system, and techniques such as file caching and stateless file servers that are used to ensure good performance and reliability, respectively.
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Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 1 Copyright © 2008 Design issues in Distributed File Systems • Transparency of a file system – A user need not know the location of a file in the system * Location transparency: The name of a file should not reveal its location Provides user convenience * Location independence: File system should be able to change the location of a file without having to change its name Enables the file system to optimize its own performance Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 2 Copyright © 2008 Design issues in Distributed File Systems • Fault tolerance – Two techniques are used to ensure that a fault does not disrupt operation of a file system * Journaling technique may be used to ensure consistency of meta data * Stateless file server design eliminates the need to maintain consistency of meta data Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 3 Copyright © 2008 Design issues in Distributed File Systems • Performance – File system performance has two aspects * High efficiency File caching boosts efficiency by reducing network traffic * Scalability Response time should not degrade as system size grows Special scalability techniques are employed » Clusters of computers » Distributed locking techniques Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 4 Copyright © 2008 Basics of file processing in a DFS • • • • A user or process that accesses a file is called a client When the client opens a file, the DFS finds its location during name resolution DFS sets up the arrangement involving the client and file server agents This arrangement is analogous to a remote procedure call (RPC) Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed SlideNo:5 Copyrightâ2008 Transparency DFS may use the following arrangement – Each file is assigned a globally unique file id * It is stored in the directory entry of the file – DFS uses a separate data structure to hold file id-location pairs * If the DFS changes a file’s location, it updates its entry in this data structure Chapter 19: DistributedFileSystems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:6 Copyrightâ2008 File sharing semantics Specify how file updates are visible to concurrent users – Unix semantics * Use single image mutable files Updates made by one client are immediately visible to others – Session semantics * Employ multiple image mutable files Clients in a session use a single image mutable file » Their updates are visible to one another immediately but are visible to other clients only when the session ends – Transaction semantics * File processing performed by a client is implemented as an atomic transaction, so only one client can access a file at any time * Provides reliability when faults occur Chapter19: DistributedFileSystems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:7 Copyrightâ2008 Session semantics Only clients in the same node can form a session • A new session is started in a node if no session is in progress in it, or if some client of a previous session has updated the file and performed a close operation • The semantics not guarantee portability because they not specify which file version should be accessed by a new client Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 8 Copyrightâ2008 Fault tolerance File system reliability has many facets A file should be – Robust * A file must survive faults in a guaranteed manner Techniques such as disk mirroring ensure robustness – Recoverable * It should be possible to restore it to an earlier state when a fault occurs – Available * It should be accessible despite faults Availability is typically ensured while opening a file Ensuring it during file processing would require a complex arrangement (see quorum algorithms in Chapter 18) Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:9 Copyrightâ2008 Fault tolerance techniques Three key techniques employed for fault tolerance – Cached directories * A remote directory is cached during name resolution helps in tolerating failures of intermediate nodes in future name resolutions – File replication * Quorum algorithms can be used to control read / write accesses; however, they are expensive Instead, only the primary copy of a file may be updated and the updates are propagated to other copies – Stateless file server * Does not maintain any meta data Hence immune to consistency problems when faults arise Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:10 Copyrightâ2008 Stateless file servers A conventional file system performs file processing in a stateful manner – It maintains information about the activity in memory * For example, file control block, file map table, file buffers • – Loss of this information disrupts file processing activities Stateless file server – Does not maintain any information about the activity in memory * Extra work may be performed to obtain relevant information such as FMT at every file operation To avoid it, file server may return some information to the client process, which can provide it at the next operation Hint: Server maintains some of the information and uses it if available; otherwise, it acts as a stateless server Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 11 Copyright © 2008 Performance techniques of distributed file systems • Three classes of techniques – Efficiency of file access * Multi-threaded file server Threads can service file access requests concurrently * Hint-based file server Hybrid of stateful and stateless file server Uses a hint if available, else functions as a stateless server – File caching * Reduces network traffic by caching portions of a file (see next slide) – Scalability * Clusters of nodes using high speed LANs are used Processes in a cluster rarely need to access outside files Hence network traffic does not grow with number of clusters Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 12 Copyright © 2008 A schematic of file caching • • • • Cache manager in client node traps file accesses whose data exists in the cache Cache validation traffic is needed to ensure validity of cached data Client-initiated validation is performed by the cache manager in client site In server-initiated validation, server keeps track of copies of data in the caches and invalidates the copies when data is updated Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 13 Copyright © 2008 Sun Network File System • Provides a stateless file server for files on a network – Virtual file system (VFS) layer * It uses the mount protocol and creates system-wide unique file ids * Export list of a node identifies a local directory and which remote nodes can access it * Permits cascaded mounting of file systems, i.e., mounting over a mounted file system – Network file system (NFS) layer * Uses a directory names cache to perform path name resolution * Uses the NFS protocol to provide access to a remote file using RPC NFS employs file caching to provide high performance Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 14 Copyright © 2008 Architecture of the Sun network file system (NFS) • The VFS interface either invokes a local file system or the NFS • NFS uses the RPC protocol to implement file operations • VFS interface acts as both a client and a server Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 15 Copyright © 2008 Andrew and Coda file systems • Features – Scalable performance is obtained through use of clusters and caching of whole files on nodes within a cluster – Files of a user exist in one volume * Volumes can be mounted and migrated within the system – File sharing semantics * File is cached in units called chunks; chunk size is adapted * Server-initiated cache validation using callbacks Node caching a file F has a callback on F Callback is broken when F is updated – Features of Coda * Provides replication using read-one-write-all policy * Supports disconnected mode of operation Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:16 Copyrightâ2008 General parallel file system (GPFS) File system for clusters operating under Linux – Uses data striping; uses large block size to reduce seek overhead and small block size for high transfer rate for small files – Uses locking to maintain consistency of file data * Lock granularity is adjusted to trade-off overhead and concurrency Implemented using lock token with an adjustable byte-range – Uses partitioned free disk space map to provide concurrency – Each node writes a journal for recovery – A failure may partition the cluster * Only majority partition can continue file processing activities Prevents file inconsistency due to concurrent updates Chapter 19: DistributedFileSystems Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:17 Copyrightâ2008 Windows Features for replication and distribution – Remote differential compression (RDC) * Reduces the replication and file coherence traffic between servers Replication is performed using the notion of replication group RDC protocol synchronizes copies of a replicated folder by transmitting changes in it – DFS namespaces enables list of virtual tree of folders that can be accessed by a client located in any node * System administrator associates a list of servers with a folder * This list is given to the client that wishes to access the folder The client contacts the servers in this order to access the folder This scheme permits a hot standby arrangement Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 18 Copyright © 2008 ... maintains some of the information and uses it if available; otherwise, it acts as a stateless server Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed... server-initiated validation, server keeps track of copies of data in the caches and invalidates the copies when data is updated Chapter 19: Distributed File Systems Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed... uses a separate data structure to hold file id-location pairs * If the DFS changes a file’s location, it updates its entry in this data structure Chapter1 9: DistributedFileSystems Dhamdhere: OperatingSystems