TAILIEUCHUNG - Operating System Concepts ppt (16)
Chapter 16 Distributed-File Systems Background. Naming and Transparency. Remote File Access. Stateful versus Stateless Service. File Replication. Example Systems. Silberschatz, Galvin and Gagne 2002Background Distributed file system (DFS) – a distributed. implementation of the classical time-sharing model of a. file system, where multiple users share files and storage. resources. A DFS manages set of dispersed storage devices Overall storage space managed by a DFS is composed. of different, remotely located, smaller storage spaces. There is usually a correspondence between constituent. storage spaces and sets of files Silberschatz, Galvin and Gagne 2002DFS Structure Service – software entity running on one or more machines and. providing a particular type of function to a priori unknown clients. Server – service software running on a single machine. Client – process that can invoke a service using a set of. operations that forms its client interface. A client interface for a file service is formed by a set of primitive. file operations (create, delete, read, write). Client interface of a DFS should be transparent, ., not. distinguish between local and remote files Silberschatz, Galvin and Gagne 2002Naming and Transparency Naming – mapping between logical and physical objects. Multilevel mapping – abstraction of a file that hides the. details of how and where on the disk the file is actually. stored. A transparent DFS hides the location where in the. network the file is stored. For a file being replicated in several sites, the mapping. returns a set of the locations of this file’s replicas; both. the existence of multiple copies and their location are. hidden Silberschatz, Galvin and Gagne 2002Naming Structures Location transparency – file name does not reveal the file’s. physical storage location File name still denotes a specific, although hidden, set of physical. disk blocks Convenient way to share data Can expose correspondence between component units and. machines. Location independence – file name does not need to be. changed when the file’s physical storage location changes Better file abstraction Promotes sharing the storage space itself Separates the naming hierarchy form the storage-devices. hierarchy Silberschatz, Galvin and Gagne 2002Naming Schemes — Three Main Approaches. Files named by combination of their host name and local. name; guarantees a unique systemwide name. Attach remote directories to local directories, giving the. appearance of a coherent directory tree; only previously. mounted remote directories can be accessed. transparently. Total integration of the component file systems A single global name structure spans all the files in the. system If a server is unavailable, some arbitrary set of directories on. different machines also becomes unavailable Silberschatz, Galvin and Gagne 2002Remote File Access Reduce network traffic by retaining recently accessed. disk blocks in a cache, so that repeated accesses to the. same information can be handled locally. If needed data not already cached, a copy of data is brought. from the server to the user Accesses are performed on the cached copy Files identified with one master copy residing at the server. machine, but copies of (parts of) the file are scattered in. different caches Cache-consistency problem – keeping the cached copies. consistent with the master file Silberschatz, Galvin and Gagne 2002Cache Location – Disk vs. Main Memory Advantages of disk caches. More reliable C
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