File Systems (original) (raw)

Table of Contents

Definition

• Reference: S&G, Chapter 10
• file system: a collection of files stored either on disk or tape
• maintained by a part of the operating system called the file system manager
• Features of a File System
  • may contain large amounts of information
  • persistent: information is retained even when no process is using it
  • a file system can be accessed by many processes at once, where many processes are between starting and finishing using files

Two Views of the File System

• Logical View (User View): abstraction of the file system presented to the user
• Physical View: how this abstraction is provided by the file manager using disks

Logical View of a File System

a)

commands: create, delete, open, close, read, write, append, seek

b)

names for files: main.c , file.bak

c)

structure of the file system

i)

flat directory (one directory)

ii)

two-level directory

iii)

tree-structured directory (hierarchial directory)

iv)

acyclic graph directory structure (no cycles)

So, what happens when you use cd .. command?

Types of Linking

a)

symbolic linking

• slower in use
• can link to a file whose content changes
• can link across file systems (and machines under NFS)
• In UNIX, adding a symbolic link has no effect on the original file and in particular has no effect on the linkcount
• if person A creates a file and person B links to it, when A deletes it, the link fails when B tries to use it
• Example:
  % ln -s /usr/w/h usr/local/h
  % rm /usr/w/h/
  % cat /usr/local/h
  UX🐈 ERROR: Cannot open /usr/local/h: No such file or directory

b)

hard link

• creates a new ``true name'' for the file
• the file is only removed when no links to it remain
• you create the file, set linkcount (stored in i-node) = 1
• someone links to the file so linkcount = 2
• you delete the file, so linkcount = 1 but the file is kept
• complication: you own a file that you no longer have any access to
• Example:
  % ln /usr/w/h /usr/local/h
  % rm /usr/w/h
  % cat /usr/local/h # still works because file still exists

Access Control (File Protection)

• Reference: S&G, Section 10.4 and Chapter 13
• each domain defines a set of objects (i.e., files) and the types of operations that may be performed on each object
• in the simplest case, a domain is a user

Some Privileges

Example Access Matrix

Other Privileges from S & G

Capability List

• for every domain, list the objects and their privileges (only objects for which the subject has some privileges)

/fred/mail /fred/prog /fred/prog.c
fred

/fred/mail /sam/loveletter /fred/prog
sam

/fred/mail /fred/prog
kelly

/fred/mail /sam/loveletter /fred/prog.c
marg

Access List

• for every object, list the domains and their privileges (only objects for which the subject has some privileges)

fred sam kelly marg
/fred/mail

sam marg
/sam/loveletter

fred sam kelly
/fred/prog

fred marg
/fred/prog.c

In UNIX:

• We classify the users into three catagories.
• Thus, from each user's perspective, there are three domains.
• Simplified access list:
  • u = user (me)
  • g = others in my group (group)
  • w = any other user (world) me mygroup others
    file

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