Table of Contents
File Systems
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:cat: 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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