Transcript Operating Systems 10. File System Interface (Ch. 10 S&G) Objectives

Operating Systems
Certificate Program in Software Development
CSE-TC and CSIM, AIT
September -- November, 2003
10. File System Interface
(Ch. 10 S&G)
 Objectives
– describe the user interface to the file
system (files, directories, access)
OSes: 10. FileSys Intf.
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Contents
1. The File Concept
2. Access Methods
3. Directory Structure
4. Protection
5. Consistency Semantics
OSes: 10. FileSys Intf.
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1. The File Concept
 The
file system provides a uniform logical
view of physical storage.
 Smallest
logical storage unit is the file
– usually mapped to storage on a nonvolatile
physical device
OSes: 10. FileSys Intf.
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1.1. File Attributes
 Name
 Type
 Location
 Size
 Protection
– access controls for who can read,
write, execute the file
 Time(s)
OSes: 10. FileSys Intf.
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1.2. File Operations
 View
a file as an abstract data type
– data as bits/bytes/lines/records
– operations
 Six
basic operations:
– create, write, read, seek in a file, delete,
truncate
OSes: 10. FileSys Intf.
continued
5
 Requires
read and write pointers, or a
current-file-position pointer.
 Other
possible operations:
– append, copy, rename
– get/set file attributes
– etc.
OSes: 10. FileSys Intf.
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1.3. Opening a File

open() finds the file and stores a pointer to
it in an open file table (OFT).
 I/O
operations use the pointer rather than
the file name so there is no need to search
for the file each time.
OSes: 10. FileSys Intf.
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Multi-user Variation
 Several
users may have the same file open
at the same time.
 Use
two levels of internal tables:
– an OFT per process storing details on the
open files of the process
 e.g.
file pointers, access rights
– a system-wide OFT storing
process-independent details on the files
 location
OSes: 10. FileSys Intf.
on disk, size, open count, lock(s)
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Two-level File Tables
VUW CS 305
OFT
process P1
OFT
system OFT
process P2
OSes: 10. FileSys Intf.
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1.4. File Types
 Useful
so the OS can automatically modify
its processing behaviour
– e.g. differentiate between source code (open
with an editor) and object code (execute)
– know that files are ‘related’
e.g. example.c and example.o
OSes: 10. FileSys Intf.
continued
10
 Approaches:
– file name extensions (e.g. Windows)
– creator attributes (e.g. Mac)
– magic numbers (e.g. UNIX)
OSes: 10. FileSys Intf.
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1.5. File Structures
 Most
modern OSes support a minimal
number of file structures directly
– e.g. UNIX sees every file as a sequence of
8-bit bytes
 Benefits:
– applications have more flexibility
– simplifies the OS
OSes: 10. FileSys Intf.
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Internal File Structures
 Packing
is required to convert between
logical records and physical blocks
– internal fragmentation will occur
Logical records
Physical blocks
OSes: 10. FileSys Intf.
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2. Access Methods
 2.1.
Sequential Access
Fig. 10.3, p.347
current
position
beginning
end
rewind
read or write
OSes: 10. FileSys Intf.
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2.2. Direct Access
 A file
is made up of fixed length logical
records (a disk model).
 Can
move quickly to any record location by
supplying a relative record number
– relative to the current record position
– e.g.
seek(20);
// move to rec. 20
seek(-1);
// move to rec. 19
read();
OSes: 10. FileSys Intf.
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2.3. Indexed Access
Fig. 10.5, p.349
 Make
an index file for the file, which
contains pointers to various records
– improves search time
index Adams
file Arthur
Asher
:
:
Smith
Smith, John 007
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direct access file
OSes: 10. FileSys Intf.
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2.4. Memory Mapping
 Map
sections of a file into virtual memory.
 Reads
and writes to the mapped region act
as reads and writes to the file.
 Useful
way of sharing a file
– but requires a mutual exclusion mechanism
OSes: 10. FileSys Intf.
continued
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Memory Mapping Diagram
file
region
region
Process B
virtual memory
Process A
virtual memory
physical memory
OSes: 10. FileSys Intf.
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3. Directory Structure
 Partitions
(mini-disks, volumes)
– provide separate logical spaces on one disk
– group several disks into a single logical space
 Device
directory
– holds file information (e.g. name, location, size,
type) for all files in that partition
OSes: 10. FileSys Intf.
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Typical Directory Operations
 Search
 Create
a file
 Delete a file
 List a directory
 Remove a file
 Traverse all the files
– e.g. for making backups
OSes: 10. FileSys Intf.
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Types of Directory Structure
 3.1.
Single-level Directory
 3.2.
Two-level Directory
 3.3.
Tree-structured Directory
 3.4.
Acyclic Graph Directory
 3.5.
General Graph Directory
OSes: 10. FileSys Intf.
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3.1. Single-level Directory
Fig. 10.7, p.351
 Easy
to support and understand.
 Problems start when there are large
numbers of files and/or users.
log
OSes: 10. FileSys Intf.
foo
recs1
test
data
mail
recs2
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3.2. Two-level Directory Fig. 10.8, p.352
user1
user2
Master File
user3 Directory (MFD)
User File
Directory
(UFD)
log
OSes: 10. FileSys Intf.
foo
recs1
test
data
mail
recs2
continued
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Some Issues
 How
isolated are users?
 How
is a path defined?
– e.g. /user1/foo
 How
do users access system files?
– copying
– (extendible) search paths
OSes: 10. FileSys Intf.
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3.3. Tree-structured Directory
Fig. 10.9, p.354
root
dict
list
all
w
count words
count
list
OSes: 10. FileSys Intf.
spell
rade
list
w7
continued
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 Treat
a subdirectory like another file
– use a special bit in the directory entry to
distinguish a file (0) from a subdirectory (1)
 Absolute
– e.g.
 How
OSes: 10. FileSys Intf.
vs. relative path names?
/spell/words/rade
../spell/words/rade
is a non-empty subdirectory deleted?
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3.4. Acyclic Graph Directory
Fig. 10.10, p.357
root
dict
list
all
w
count words
count
list
OSes: 10. FileSys Intf.
spell
rade
list
w7
continued
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 A natural
generalisation of a tree-structured
directory scheme
– allows files/subdirectories to be shared
 How
are cycles avoided?
 A file/subdirectory
may have multiple
absolute path names
– complicates traversal
OSes: 10. FileSys Intf.
continued
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 How
are shared files/subdirectories deleted?
– leave ‘dangling pointers’ (cheap)
– use an access list (expensive)
– use a reference count
OSes: 10. FileSys Intf.
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Sharing in UNIX
 Symbolic
links
– a pointer to another file/subdirectory
– easily identified by a bit set in the file entry
– deletion leaves links ‘dangling’
 Hard
links
– keep a reference count for hard links to a file
– cannot link to a subdirectory
 avoids
OSes: 10. FileSys Intf.
cyclic graphs
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3.5. General Graph Directory
Fig. 10.10, p.357
root
avi
text
mail
count
jim
book
book
avi
OSes: 10. FileSys Intf.
mail
count
continued
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 Traversal
could go into an infinite loop
– use a bounded search
 A subdirectory
that refers to itself will never
have a reference count of 0
– not possible to delete it
 Garbage
collection is required to reclaim
inaccessible files/subdirectories
– very expensive
OSes: 10. FileSys Intf.
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4. Protection
 Protection
mechanisms control/limit file and
directory access operations, such as:
– reading, writing, execution,
appending, deletion, listing
 Protection
levels and types depend on the
system
– PC --> corporate installation
OSes: 10. FileSys Intf.
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4.1. Access Lists & Fields
 Access
List
– specify access rights for every user of every file
– tedious; leads to very large lists
 Fields
(Groups)
– owner/group/world
– each field in UNIX has 3 bits for read, write,
and execute permissions
OSes: 10. FileSys Intf.
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UNIX Example

bazooka<ad>47: ls
total 62
drwxr-xr-x 2 ad
-rw-r--r-- 1 ad
-rw-r--r-- 1 ad
-rw-r--r-- 1 ad
drwxr-xr-x 2 ad
-rw-r--r-- 1 ad
drwxr-xr-x 2 ad
-rwxr-xr-x 1 ad
-rw-r--r-- 1 ad
-rw-r--r-- 1 ad
OSes: 10. FileSys Intf.
Fig. 10.12, p.364
-l
512
14109
1878
782
512
24450
512
11676
4150
887
Nov
Nov
Nov
Nov
Feb
Nov
Nov
Feb
Dec
Nov
4 1998 Figures/
12 1998 Listings
4 1998 abstract.tex
4 1998 cover-note
16 08:59 old_code/
4 1998 progHTTP.txt
12 1998 revisted/
16 08:52 sock_brow*
22 17:07 sock_brow.c
4 1998 web-tech-addr
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4.2. Passwords
 A password
per file
– too hard to remember
 A password
per subdirectory
– too course-grained
OSes: 10. FileSys Intf.
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5. Consistency Semantics
 Consistency
semantics is how an OS deals
with modifications to a shared file by
multiple users who are accessing the file at
the same time.
OSes: 10. FileSys Intf.
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Consistency Sems for UNIX
 A write
is immediately visible to every
shared users.
 Processes
may share the current position
pointer of the file.
 Users
see a shared file as representing a
single physical entity.
OSes: 10. FileSys Intf.
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Consistency Sems for Andrew FS
 A distributed
file system from CMU.
 A write
is not immediately visible.
 A change becomes visible to subsequent
opens after the file has been closed.
 Users
see a shared file as representing
several temporary (possibly different)
images of the single physical entity.
OSes: 10. FileSys Intf.
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Immutable Shared Files
 A type
of file which cannot be modified if
it is declared as shareable by its creator.
 Greatly
simplifies sharing in a distributed
file system.
OSes: 10. FileSys Intf.
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