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CS 4410 Operating Systems
File-System Interface
Summer 2013 Cornell University
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Today
- Why do we use files and how are they
- rganized?
- File
- Attributes
- Operations
- Directories
- Logical structure of directory
- File protection
File-System Interface Summer 2013 Cornell University 1 Today Why - - PowerPoint PPT Presentation
File-System Interface Summer 2013 Cornell University 1 Today Why - - PowerPoint PPT Presentation
CS 4410 Operating Systems File-System Interface Summer 2013 Cornell University 1 Today Why do we use files and how are they organized? File Attributes Operations Directories Logical structure of directory File
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Organizing Data
- Every user has much useful data.
- Text, images, music, code
- Data is stored in storage devices.
- Magnetic Disk, Tape, USB
- How data is organized in storage devices?
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File
- The OS defines the File.
- Logical storage unit
- Abstraction
- The same abstraction for all storage devices.
- Different implementation.
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File
- A named collection of related information.
- Recorded on secondary storage.
- The smallest allotment of logical secondary
storage.
- Each file has a structure.
- Independent entity.
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File Attributes
- How do we identify a file?
- Name
- Type
- Location
- Size
- Protection
- Time, date and user identification
- Where are they saved?
- Directory structure
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File Operations
- What can we do with files?
- Create
- Write
- Read
- Reposition
- Delete
- Truncate
- What about the constant searching of the file?
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Open File
- An open() system call is made before a file is used.
- It saves the file information into the open-file table of the OS.
- The open() returns a pointer to the corresponding entry of the
- pen-file table.
- Subsequent accesses do not search the directory, but they get
the information needed from the pointer.
- What happens when multiple processes open the same file?
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Open File
- The OS uses two levels of internal tables:
- Per-process table
- System-wide table
- Per-process table:
- One entry for each open file (by the process).
- Stores information concerning the specific use of the file by the specific
process.
- Current file pointer, access rights
- Every entry has a pointer to the corresponding entry in the System-wide table.
- System-wide table:
- One entry for each open file (by all the processes)
- Contains process-independent information
- Location, access dates, size
- What happens when a process opens a file?
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Access Methods
- After we open the file, how can we access it?
- Sequentially
- Directly
- A file is made up of fixed-length logical records.
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File System
- How the files are organized in the secondary storage device?
- A file belongs to a File System.
- A File System lives in a volume.
- A volume is a subset of the device storage space.
- A File System consists of files and directories.
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Logical Structure of a Directory
- Single-Level
- All the files in one directory.
- Limitations?
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Logical Structure of a Directory
- Two-Level
- User file directory
- Master file directory
- Path name for every file.
- Limitations?
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Logical Structure of a Directory
- Tree-Structured
- A directory is, simply, another file.
- It is treated in a special way.
- One bit distinguishes files from directories.
- Limitations?
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Logical Structure of a Directory
- Acyclic-Graph
- Shared directories/files
- Link
– Symbolic
- The file contains the path of the shared file.
- Can span different file systems.
– Hard
- The file entry shows directly to the shared file.
- More efficient.
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File Protection
- Type of Access
- Read
- Write
- Execute
- Append
- Delete
- List
- Access Control List
- Who can access what and in what way.
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Today
- Why do we use files and how are they
- rganized?
- File
- Attributes
- Operations
- Directories
- Logical structure of directory
- File protection