6.0 FILE SYSTEM MANAGEMENT

File System

o Also referred to as simply a file system or filesystem.

o The system that an operating system or program uses to organize and keep track of files.

o For example, a hierarchical file system is one that uses directories to organize files into a tree structure. 

o Although the operating system provides its own file management system, you can buy separate file management systems.

o  These systems interact smoothly with the operating system but provide more features, such as improved backup procedures and stricter file protection. 

File Sharing

o Sharing of files on multi-user systems is desirable

o Sharing may be done through a protection scheme

o On distributed systems, files may be shared across a network

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

File Sharing – Multiple Users

o User ID's identify users, allowing permissions and protections to be per-user

o Group ID's allow users to be in groups, permitting group access rights

File Sharing – Failure Modes

o Remote file systems add new failure modes, due to network failure, server failure

o Recovery from failure can involve state information about status of each remote request

o Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security

File Sharing – Consistency Semantics

o Consistency semantics specify how multiple users are to access a shared file simultaneously

ú Similar to process synchronization algorithms

•       Tend to be less complex due to disk I/O and network latency (for remote file systems)

ú Andrew File System (AFS) implemented complex remote file sharing semantics

ú Unix file system (UFS) implements:

•       Writes to an open file visible immediately to other users of the same open file
•       Sharing file pointer to allow multiple users to read and write concurrently

ú AFS has session semantics

•      Writes only visible to sessions starting after the file is closed

File-system Organization

Operations Performed on Directory

o Search for a file

o Create a file

o Delete a file

o List a directory

o Rename a file

o Traverse the file system

Single-Level Directory

o A single directory for all users

Naming problem

Grouping problem

Two-Level Directory

o Separate directory for each user

o Path name

o Can have the same file name for different user

o Efficient searching

o No grouping capability

Tree-Structured Directories

o Efficient searching

o Grouping Capability

o Current directory (working directory)

o cd /spell/mail/prog

o type list

File Map

o File mapping is the association of a file's contents with a portion of the virtual address space of a process.

o The system creates a file mapping object (also known as a section object) to maintain this association.

o A file view is the portion of virtual address space that a process uses to access the file's contents. 

o File mapping allows the process to use both random input and output (I/O) and sequential I/O.

o It also allows the process to work efficiently with a large data file, such as a database, without having to map the whole file into memory.

o Multiple processes can also use memory-mapped files to share data.

o Processes read from and write to the file view using pointers, just as they would with dynamically allocated memory.

o The use of file mapping improves efficiency because the file resides on disk, but the file view resides in memory.

o Processes can also manipulate the file view with the VirtualProtect function.

Open File Locking

o Provided by some operating systems and file systems

o Mediates access to a file

o Mandatory or advisory:

o Mandatory – access is denied depending on locks held and requested

o Advisory – processes can find status of locks and decide what to do