Chapter 12 File Management Systems

Chapter 12

File Management Systems

Outline

Functions and components Storage allocation File manipulation Access controls Migration, backup, recovery


Logical and Physical Storage Views File Content and Type


A File Management System (FMS) is implement in layers: Command layer or application program File control Storage I/O control Storage devices


Storage devices – interact with the bus and with operating system device drivers to transfer data between storage devices and memory.

Storage I/O control – accesses storage locations and manages data movement between storage devices and memory.


File Control – provides a set of service functions for manipulating files and directories.

Command layer or application program – users perform common file management functions such as copying, moving and renaming.


Logical and Physical Storage Views

Logical Storage Views – viewed by users are a collection of files organized within directories and storage volumes.

Physical Storage Views – a collection of physical storage locations organized as a linear address space.


Records and Fields


File Content and Type

A file can store many different data types including text, numbers, complex data structures, and executable instructions.

Modern file management systems provide a framework to support additional file types.


File Content and Type

File type normally is declared when a file is created.

In the UNIX file management system, the file type is stored within the directory. In the Windows file management system, the file type is declared through the extension.


Directory Content and Structure

Hierarchical Directory Structure Graph Directory Structure

Directory Content and StructureHierarchical Directory Structure

A directory contains information about files and other directories.

Typical directory contents include: Name Size Time stamp File Type Ownership Location Access controls

Directory Content and Structure Typical file ownership permissions are:

List Read Modify Change

Directory Content and Structure Time stamps include:

When the file was created When the file most recently was read When the file most recently was written When the file last was backed up


Directory Content and StructureHierarchical Directory Structure

Directories can contain other directories. Directories can not have more than one

parent. Sometimes called a tree structure.


Directory Content and Structure Example: Traversing directories in

the Windows command shell


Graph Directory Structure

Files and subdirectories can be contained within multiple directories.

Directory links can form a cycle.


Paths

Where do operating systems look for executable files? In the current directory In the path

If directories are missing or in the wrong order, OS can’t find the right files

Example: Windows PATH

Paths and Environment Variables

Some programs use environment variables to store path information

If directories are missing or in the wrong order, programs can’t find the right files

Example: Java and the CLASSPATH

Paths, INI Files, Programs …

Paths are all over the place! They are in initialization files,

programs, the Windows registry, … You need to be comfortable with

absolute and relative file specs.

Outline


Storage Allocation

Allocation Units Storage Allocation Tables Blocking and Buffering

Storage Allocation

Allocation Units

An allocation unit is the smallest number of secondary storage bytes that can be allocated to a file.

Allocation units can not be smaller than the unit of data transfer between the storage device and controller, which is normally called a block.

Systems Architecture Chapter 12

Storage Allocation

Allocation unit size is usually a tradeoff among:

Efficient use of secondary storage space for files. Size of storage allocation data structures. Efficiency of storage allocation procedures.


Storage Allocation

Storage Allocation Tables

A storage allocation table is a data structure that records which allocation units are free and which belong to files.


Storage Allocation

Outline

Functions and components Storage allocation File manipulation Migration, backup, recovery Access controls Directories

Storage Allocation

Storage Allocation

Storage Allocation

Blocking and Buffering

A logical record is a collection of data items, or fields, that is accessed by an application program as a single unit.

A physical record is the unit of storage transferred between the device controller and memory in a single operation.

Storage Allocation

Blocking and Buffering

Logical record grouping within physical records is called blocking.

If a physical record contains just one logical record, then the file is said to be unblocked.

Storage Allocation

Storage Allocation

Blocking and Buffering:

A FMS uses buffers in primary storage to store data temporarily as it moves between programs and secondary storage devices.

A buffer is a scratchpad for extracting logical records from physical records.

Storage Allocation

Outline


File Manipulation

File Open and Close Operations Delete and Undelete Operations

File Manipulation

File Open and Close Operations

The FMS must perform several tasks, collectively called a file open operation, before an application program can read or write a file’s contents.

File Manipulation

File Open Operation:

1. Locates the file within the directory structure and reads its directory entry.

2. Searches an internal table of open files to see if the file already is open.

3. Ensures that process has sufficient privileges to access the files.

4. Allocates one or more buffers.5. Updates an internal table of open files.

File Manipulation

File Close Operation:

1. Flushing the program’s file I/O buffers to secondary storage.

2. Deallocating buffer memory.

3. Updating the file’s directory entry time stamps.

4. Updating the open file table.

File Manipulation

Delete and Undelete Operations

In most file management systems, files are not removed immediately from secondary storage when they are deleted.

The file’s storage allocation units are marked as free and its directory entry is marked as unused.

A user might be able to use the undelete operation to recover the file.

Access Controls

A File Management System helps prevent loss, corruption and unauthorized access to files.

The operating system is used to identify and authenticate users and their processes.

The file access is authenticated through id’s and passwords.

Outline


Access Controls

For Example:

UNIX defines three access control types: Read Write Execute

Technology Focus – Windows NTFS

Outline


File Migration, Backup and Recovery File Migration File Backup Transaction Logging File Recovery Fault Tolerance Mirroring

File Migration, Backup and Recovery

File Migration – management technique for secondary storage in which older versions of a file are moved automatically to less costly storage media or devices such as magnetic tape.

File Migration


File Backup:

Full Backup

Incremental Backup

Differential Backup


Full Backup – the FMS copies all files and directories for an entire storage volume.

Incremental Backup – only the files that have been modified are archived.

Differential Backup – only the changed portions of the files are archived.


Transaction Logging

A form of automated file backup. A transaction is any single change to file

contents or attributes. Transaction logging provides a high degree of

protection against data loss due to program or hardware failure.


File Recovery

The file management system maintains backup logs to aid in locating backup copies of lost or damages files.

The recovery utility reconstructs as much of the directory and storage allocation data structures as possible and makes a consistency check.


Fault Tolerance

Describes methods of securing file content against hardware failure.

File backup, recovery, and transaction logging are forms or protection against disk failure.


Mirroring

A fault tolerance technique in which all disk write operations are made simultaneously or concurrently to two different storage devices.

Disk mirroring provides a high degree of protection against data loss with no performance penalty if implemented in hardware.


Technology Focus

Technology Focus

Technology Focus

Summary

The file management system (FMS), usually a part of the operating system, manages all aspects of user and program access to secondary storage.

With directories, users can organize the thousands of files stored in a typical computer system.

Summary

Secondary storage units are divided into allocation units, which are typically a few kilobytes in size.

The FMS allocates buffers to support program file I/O.

Summary

The FMS enforces access controls when accessing files on behalf of a user or program.

FMSs provide utilities to make backup copies of files and directories and to recover them if needed.

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Chapter 12 File Management Systems