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File System Interface
CSCI 444/544 Operating Systems
Fall 2008
Agenda
- File system abstraction
- Directory structure
- File operations
- File system mounting
File System Abstraction
File system is the OS abstraction for storage resources
(especially, disk)
Track/sector <=> files
- Provides a uniform logical view of information storage
- File is a logical storage unit in the OS abstract interface
for storage resources
- Directory is a logical “container” for a group of files
File Concept
To a user process, a file is a contiguous block of bytes
User view: named collection of bytes
OS view: collection of blocks on physical non-volatile
storage device
Types:
- Data
- Program (source and object)
File Structure
Sequence of words, bytes
Simple record structure
- Lines
- Fixed length
- Variable length
Complex Structures
File Attributes (Meta-data)
Name – a string used to uniquely identify a file
Identifier – unique tag (number) identifies file within file
system
Type – needed for systems that support different types
Location – 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
These meta-data (Information about files) are kept in the
directory structure, which is maintained on the disk but
cached in memory if the file is open.
File Systems
OK, we have files
How can we name them?
How can we organize them?
File Naming
Each file has an associated human-readable name
- e.g., usr, bin, mid-term.pdf, design.pdf
OS must maintain a mapping between file names
and the set of blocks belong to that file
- In Unix, this is a mapping between names and i-nodes
Mappings are kept in directories
Two-Level Directory
Separate directory for each user
Path name
Can have the same file name for different user
Efficient searching
No grouping capability
Tree-Structured Directories
Tree-Structured Directories (Cont)
Efficient searching
Grouping Capability
Current directory (working directory)
- cd /spell/mail/prog
- type list
Tree-Structured Directories (Cont)
Absolute or relative path name
Creating a new file is done in current directory
Delete a file
rm <file-name>
Creating a new subdirectory is done in current
directory
mkdir <dir-name>
Example: if in current directory /mail
mkdir count
mail
prog copy prt exp count
Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”
Acyclic-Graph Directories
Have shared subdirectories and files
Acyclic-Graph Directories (Cont.)
More general than tree structure
- Add connections across the tree (no cycles)
- Create links from one file (or directory) to another
Two different names (aliasing)
New directory entry type
- Link – another name (pointer) to an existing file
- Resolve the link – follow pointer to locate the file
File Operations
Create
Write
Read
Reposition within file
Delete
Truncate
Open(F
i
) – search the directory structure on disk for
entry F
i
, and move the content of entry to memory
Close (F
i
) – move the content of entry F
i
in memory
to directory structure on disk
Open Files
Open() file before first access
- User specifies mode: read and/or write
- Search directories for filename and check permissions
- Copy relevant information to open file table in memory
- Return index in open file table to process (file descriptor)
- Process uses file descriptor to read/write to file
Two levels of open file tables
- Per-process open file table
- Track all files that a process has open
- Current file pointer indicating the current read or write position
- System-wide open-file table
- Contains process-independent file information
- Location of the file on disk, file size, etc.
