File System Implementation

Chapter 6.3Thursday, April 19, 2007

Today’s Schedule Files

Overview of files & directories (6.1-2) System Implementation (Chapter 6.3)

Objectives

You will be able to describe: User’s view of file and directory File abstractions provided by O/S Different file types according to how they

are laid out on disk blocks

Files: Long-term Information Storage

1. Must store large amounts of data

2. Information stored must survive the termination of the process using it

3. Multiple processes must be able to access the information concurrently

File Structure

Three kinds of filesbyte sequence record sequence tree

File Access Sequential access

read all bytes/records from the beginningcannot jump around, could rewind or back upconvenient when medium was mag tape

Random accessbytes/records read in any orderessential for data base systemsread can be …

move file marker (seek), then read or … read and then move file marker

File Attributes

Possible file attributes

File Operations – Interface Abstractions

1. Create2. Delete3. Open4. Close5. Read6. Write

7. Append8. Seek9. Get attributes10.Set Attributes11.Rename

Memory-Mapped Files

(a) Segmented process before mapping files into its address space

(b) Process after mapping existing file abc into one segment creating new segment for xyz

Shared file use problem???

DirectoriesSingle-Level Directory Systems

A single level directory systemcontains 4 filesowned by 3 different people, A, B, and C

Two-level Directory Systems

Letters indicate owners of the directories and files

Hierarchical Directory Systems

A hierarchical directory system

A UNIX directory tree

Path Names

Directory Operations1. Create2. Delete3. Opendir4. Closedir

5. Readdir6. Rename7. Link8. Unlink

File System Implementation

A possible file system layout

Contiguous Allocation

(a) Contiguous allocation of disk space for 7 files(b) State of the disk after files D and E have been removed

Linked List Allocation

Storing a file as a linked list of disk blocksSequential read is easy

Linked List Using Table in Memory

Linked list allocation using a file allocation table in RAM

I-Nodes

An example i-node

The UNIX V7 File System

A UNIX i-node

Accommodating larger files!

Implementing Directories

Where store file attributes?(a) A simple directoryfixed size entriesdisk addresses and attributes in directory entry

(b) Directory in which each entry just refers to an i-node

Handling Shared Files

File system containing a shared file

Shared Files

(a) Situation prior to linking(b) After the link is created(c)After the original owner removes the file

Let’s Try some …

Chapter 6 Problems, Pg 449Problems

#2, 6, 9, 10, 11, 12, 15

Tuesday, April 24

Read Chapter 6 File Systems6.3 - Files6.4 – Example Systems