TAILIEUCHUNG - Operating System Concepts (17)
Module 17: Distributed-File Systems.• Background.• Naming and Transparency.• Remote File Access.• Stateful versus Stateless Service.• File Replication.• Example Systems. Silberschatz, Galvin, and Gagne 1999 Background.• 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 1999 DFS 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 1999 Naming 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 1999 Naming 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 1999 Naming 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 1999 Remote 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 – Fi
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