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Wednesday, February 3, 2010

Naming and Transparency in Distributed File Systems

Naming is the mapping between logical and physical objects. In a conventional file system, it's understood where the file actually resides; the system and disk are known. In a transparent DFS, the location of a file, somewhere in the network, is hidden. File replication means multiple copies of a file; mapping returns a SET of locations for the replicas.

- Location transparency -
* The name of a file does not reveal any hint of the file's physical storage location.
* File name still denotes a specific, although hidden, set of physical disk blocks.
* This is a convenient way to share data.
* It can expose correspondence between component units and machines.

- Location independence :
* The name of a file doesn't need to be changed when the file's physical storage location changes.Dynamic, one-to-many mapping.
* Better file abstraction.
* Promotes sharing the storage space itself.
* Separates the naming hierarchy from the storage devices hierarchy.

A location independent naming scheme is a dynamic mapping, since it can map the same file name to different locations at two different times. Therefore, location independence is a stronger property than location transparency.
Most DFSs today support location transparent systems. They do not support migration and files are permanently associated with specific disk blocks.

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