File System: Introduction

------------------------------------------------------------------------------

Concept of files:

- sequential stream of bytes; (not interpreted at the lower level);
- non-volatile;


File Attributes:

- name;

- type;
	Discussion: MS OS uses file extension as types;
	UNIX: no support, except for symbolic links;

- last modified time, creation time, last use time;

- size;

- protection (ACL);
	Access control list: 	list of (user_ID or group_ID, permission);
	permission: read, write, execute, append, delete, list;

- creator ID;


(where are these kept?)



File Operations;


Referencing a file: file path name;


File descriptor table;


FS implementation: given an offset, find the block;

Tradeoff between large block size and small block size;


-------------------------------------------------------------------------------

Directories:

- a special form of file;

- tree-structure;
	. except symbolic links;

- Directory operations:

- hierarchical search of the directory;


-------------------------------------------------------------------------------

Example: the MS-DOS FAT file system.  ---- Section 11.2.2 of the book;

. File Allocation Table (FAT): blocks of a file are linked together in
	the FAT table; special markers indicate free block or end of file;
		- reading a random block in a large file is slow!
	
. Directory contains the <name, address of first block> pairs of all files
	in the directory;

. performance problems with the directory structure:
	searching for a particular file in a large directory is slow;