File System Interface CSCI 444/544 Operating Systems Fall 2008.
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Transcript of File System Interface CSCI 444/544 Operating Systems Fall 2008.
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 ConceptTo 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
– numeric– character– binary
• Program (source and object)
File Structure
Sequence of words, bytes
Simple record structure• Lines • Fixed length• Variable length
Complex Structures• Formatted document
File Attributes (Meta-data)
Name – a string used to uniquely identify a fileIdentifier – unique tag (number) identifies file within file systemType – needed for systems that support different typesLocation – pointer to file location on deviceSize – current file sizeProtection – controls who can do reading, writing, executingTime, 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 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
Directory StructureA collection of nodes containing information about all files
F 1 F 2F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on diskBackups of these two structures are kept on tapes
Operations Performed on Directory
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
Organize the Directory to Obtain
Efficiency – locating a file quickly
Naming – convenient to users• Two users can have same name for different
files• The same file can have several different names
Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)
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 (Cont)
Efficient searching
Grouping Capability
Current directory (working directory)• cd /spell/mail/prog• type list
Tree-Structured Directories (Cont)Absolute or relative path nameCreating a new file is done in current directoryDelete a file
rm <file-name>Creating a new subdirectory is done in current directory
mkdir <dir-name>
Example: if in current directory /mailmkdir count
prog copy prtexpcount
Deleting “mail” deleting the entire subtree rooted by “mail”
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
CreateWriteReadReposition within fileDeleteTruncate
Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory
Close (Fi) – move the content of entry Fi in memory to directory structure on disk
Open FilesOpen() 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.
Related Information to an Open File
Several pieces of data are needed to manage open files:• File pointer: pointer to last read/write location, per
process that has the file open• Access rights: per-process access mode information• File-open count: counter of number of times a file is
open – to allow removal of data from system-wide open-file table when last processes closes it
• Disk location of the file: cache of data access information
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Data Structures for a File
Processcontrolblock
...
Openfile
pointerarray
Open filetable
(system-wide)i-node table
Filei-node
Access Methods
Sequential Accessread nextwrite next resetno read after last write
Direct Accessread nwrite nposition to n
read nextwrite next
rewrite nn = relative block number