Ch12_OS_Lecture 4.pdf

Operating System

1. Chapter (12)
File Management
Dr. Hnin Lai Nyo
Lecturer
Faculty of Computer Science

2. Lecture Contents
Lecture 1: File Management System Overview
Lecture 2: File Organization and Access
Lecture 3: B-Tree and File Directories
Lecture 4: File Sharing and Record Blocking
Lecture 5: File Allocation and Methods
Lecture 6: Free Space Management
FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES

3. Lecture 4: File Sharing and Record Blocking
FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
File Sharing
Access Rights
Simultaneous Access
Record Blocking

4. Learning Objectives
FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Todescribetherequirementsforfilesharing.
Toexplaintheconceptofrecordblocking.

5. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Inamultiusersystem,thereisalmostalwaysarequirementforallowingfilestobe
sharedamonganumberofusers.
Whenfilesharingamongusers,twocasesappear:
AccessRights
Managementofsimultaneousaccess
File Sharing

6. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Access Rights
Thefilesystemshouldprovideaflexibletoolforallowingextensivefilesharing
amongusers.
Thefollowingareaccessrightsthatcanbeassignedtoaparticularuserfora
particularfile.
None:Theuserwouldnotbeallowedtoreadtheuserdirectorythatincludes
thefile.
Knowledge:Theusercandeterminethatthefileexistsandwhoitsowneris.
Execution:Theusercanloadandexecuteaprogrambutcannotcopyit.

7. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Access Rights
Reading:Theusercanreadthefileforanypurpose,includingcopyingand
execution.
Appending:Theusercanadddatatothefilebutcannotmodifyordeleteanyof
thefile’scontents.
Updating:Theusercanmodify,delete,andaddtothefile’sdata.
Changingprotection:Theusercanchangetheaccessrightsgrantedtoother
users.
Deletion:Theusercandeletethefilefromthefilesystem.

8. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Access Rights
Owner
Usually the initial
creator of the file
Has full rights
May grant rights
to others
Specific Users
Individual
users who are
designated by
user ID
User Groups
A set of users
who are not
individually
defined.
All
All users who
have access to
this system
These are public
files
Access can be provided to different classes of users.

9. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Simultaneous Access
Whenaccessisgrantedtoappendorupdateafiletomorethanoneuser,the
operatingsystemorfilemanagementsystemmustenforcediscipline.
Abrute-forceapproachistoallowausertolocktheentirefilewhenitistobe
updated.
Afinergrainofcontrolistolockindividualrecordsduringupdate.

10. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Record Blocking
BlocksareunitofI/Owithsecondarystorage.
ForI/Otobeperformed,recordsmustbeorganizedasblocks.
Giventhesizeofblock,therearethreemethodsofblockingthatcanbeused;
Fixed-LengthBlocking:fixed-lengthrecordsareused,andanintegralnumberof
records(orbytes)arestoredinablock.
Internalfragmentation–unusedspaceattheendofeachblock.
Variable-LengthSpannedBlocking:variable-lengthrecordsareusedandarepacked
intoblockswithnounusedspace.
Variable-LengthUnspannedBlocking:variable-lengthrecordsareused,butspanning
isnotemployed.

11. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Record Blocking
R2 R3 R4 Track 1R1
R6 R7 R8 Track 2R5
Fixed blocking
R3 R4 R6 Track 1
R1
R8 R9 R13 Track 2R6
Variable blocking: spanned
R2 R4 R5
R7 R9 R10 R11 R12
R2 R4 R5 Track 1R1
R7 R8 R10 Track 2R6
R3
R9
Data Gaps due to hardware design
Waste due to block fit to track size
Waste due to record fit to block size
Waste due block size constraint from fixed record size
Figure 12.8. Record
Blocking Methods

12. FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Summary
Inthislecture,youlearnedabout
File sharing in which access rights and access that can be provided to different
classes of users.
Three types of record blockings.

13. Learning Outcomes
FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
Aftertheendofthelecture,thestudentswillbeableto:
understandaccessrightsthatpermittoaccessordenyafileoradirectory.
knowaccessrightsthatcanbeprovidedtothedifferentclassofusers.
recognizethathowtousethreemethodsofblockingdependingonthesizeofthe
recordblocks.

14. References
FACULTY OF COMPUTER SCIENCE, UNIVERSITY OF COMPUTER STUDIES
1.“Operating Systems, internal and Design Principles” by William Stallings, 7
th
Edition, ISBN 13: 978-13-230998-1.