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File System

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File System consists of

✦ A collection of files

✦ A directory structure

✦ (possibly) partitions

Important Issues

✦ File protection

✦ The semantics of file sharing

File Concept:

• The operating system provides a uniform logical abstraction for the physical storage of information.

• Storage devices are nonvolatile.

• A file is a named collection of related information that is recorded on secondary storage.

• Contiguous logical address space

• Types:

✦ Data
✔ Numeric
✔ Character
✔ Binary
✦ Program
✔ Source, object and executable file formats

File Attributes:

Name - only information kept in human-readable form.

Identifier - a unique tag (i.e., an internal number) that identifies the file within the file system.

Type - needed for systems that support different types.

Location - a pointer to file location on device.

Size - current file size.

Protection- controls who can do reading, writing, executing.

Time, date, and user identification - data for protection, security, and usage monitoring.

• Information about files are kept in the directory structure, which is maintained on the disk.


File Sharing:

• Sharing of files on multi-user systems is desirable.

• Sharing may be done through a protection scheme.

• On distributed systems, files may be shared across a network.

• Network File System (NFS) is a common distributed file sharing method


Protection:

File owner/creator should be able to control:

✦ what can be done

✦ by whom

Types of access:

✦ Read

✦ Write

✦ Execute

✦ Append

✦ Delete

✦ List






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