Transcript Figure 9.01 - University of South Florida

Chapter 10 & 11: File-System Interface
and Implementation
Operating System Concepts – 8th Edition
14.1
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Operating System Concepts – 8th Edition
14.2
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A Typical File-system Organization
Operating System Concepts – 8th Edition
14.3
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Files




Contiguous logical address space
Types:
 Data: numeric, character, binary
 Program
Structure
 None - sequence of words, bytes
 Simple record structure
 Lines
 Fixed length
 Variable length
 Complex Structures
 Formatted document
 Relocatable load file
Who decides the structure:
 Operating system
 Program
Operating System Concepts – 8th Edition
14.4
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File Attributes
 Name – only information kept in human-readable form
 Identifier – unique tag (number) identifies file within file system
 Type – needed for systems that support different types
 Location – pointer to file location on device
 Size – current file size
 Protection – controls who can do reading, writing, executing
 Time, date, and user identification – data for protection, security,
and usage monitoring
 Information about files are kept in the directory structure, which is
maintained on the disk
Operating System Concepts – 8th Edition
14.5
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File Operations
 File is an abstract data type
 Create
 Write
 Read
 Reposition within file
 Delete
 Truncate
 Open(Fi) – search the directory structure on disk for entry Fi, and
move the content of entry to memory
 Close (Fi) – move the content of entry Fi in memory to directory
structure on disk
Operating System Concepts – 8th Edition
14.6
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Open Files
 Several pieces of data are needed to manage open files:

File pointer: pointer to last read/write location, per process that has the
file open
 File-open count: counter of number of times a file is open – to allow
removal of data from open-file table when last process closes it
 Disk location of the file: cache of data access information
 Access rights: per-process access mode information
 Open file locking

Provided by some operating systems and file systems
 Mediates access to a file
 Mandatory or advisory:

Mandatory – access is denied depending on locks held and requested

Advisory – processes can find status of locks and decide what to do
Operating System Concepts – 8th Edition
14.7
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File Types – Name, Extension
Operating System Concepts – 8th Edition
14.8
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Access Methods
 Sequential Access
read next
write next
reset
no read after last write
(rewrite)
 Direct Access
read n
write n
position to n
read next
write next
rewrite n
n = relative block number (relative to the beginning of the file)
Operating System Concepts – 8th Edition
14.9
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Directory Structure
 A collection of nodes containing information about
all files
Directory
Files
F1
F2
F3
F4
Fn
Both the directory structure and the files reside on disk
Operating System Concepts – 8th Edition
14.10
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Operations Performed on Directory
 Search for a file
 Create a file
 Delete a file
 List a directory
 Rename a file
 Traverse the file system
Operating System Concepts – 8th Edition
14.11
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Single-Level Directory
 A single directory for all users
Naming problem
Grouping problem
Operating System Concepts – 8th Edition
14.12
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Two-Level Directory
 Separate directory for each user
 Path name
 Can have the same file name for different user
 Efficient searching
 No grouping capability
Operating System Concepts – 8th Edition
14.13
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Tree-Structured Directories

Efficient searching

Grouping capability
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Current directory (working directory)
Operating System Concepts – 8th Edition
14.14
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Tree-Structured Directories (Cont)
 Absolute or relative path name
 Creating a new file is done in current directory
 Delete a file
rm <file-name>
 Creating a new subdirectory is done in current directory
mkdir <dir-name>
Example: if in current directory /mail
mkdir count
mail
prog
copy prt exp count
Deleting “mail”  deleting the entire subtree rooted by “mail”
Operating System Concepts – 8th Edition
14.15
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General Graph Directory


Links to share files – only one copy exists
How do we guarantee no cycles?
 Allow only links to files not subdirectories
 Garbage collection
 Every time a new link is added use a cycle detection algorithm to determine
whether it is OK
Operating System Concepts – 8th Edition
14.16
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File Sharing
 Sharing of files on multi-user systems is desirable
 Sharing may be done through a protection scheme

User IDs identify users, allowing permissions and protections to be per-user

Group IDs allow users to be in groups, permitting group access rights
 On distributed systems, files may be shared across a network
 Network File System (NFS) is a common distributed file-sharing method
Operating System Concepts – 8th Edition
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Protection
 File owner/creator should be able to control:

what can be done

by whom
 Types of access

Read

Write

Execute

Append

Delete

List
Operating System Concepts – 8th Edition
14.18
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Access Lists and Groups

Mode of access: read, write, execute

Three classes of users
a) owner access
7

b) group access
6

c) public access
1

RWX
111
RWX
110
RWX
001

Ask manager to create a group (unique name), say G, and add some users
to the group.

For a particular file (say game) or subdirectory, define an appropriate
access.
owner
chmod
group
public
761
game
Attach a group to a file
chgrp
Operating System Concepts – 8th Edition
G
game
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A Sample UNIX Directory Listing
Operating System Concepts – 8th Edition
14.20
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Moving-head Disk Mechanism
• Disk access costs:
Transfer time +
access time
• Access time:
– Seek time: time to
move the disk heads
to the desired cylinder
– Rotational latency:
time for rotating to
desired sector
Operating System Concepts – 8th Edition
14.21
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Files: Contiguous allocation
(a) Contiguous allocation of disk space for 7 files
(b) State of the disk after files D and E have been
removed
Operating System Concepts – 8th Edition
14.22
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Files: Linked List
The links are on the disk (using block numbers)
Operating System Concepts – 8th Edition
14.23
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Linked List: File Allocation Table
Keep the list links in a separate table in memory
Operating System Concepts – 8th Edition
14.24
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i-nodes
Operating System Concepts – 8th Edition
14.25
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A UNIX i-node
Max file size:
d direct pointers,
n indirect pointers p/block
Blocksize * (d + n + n2 + n3)
Operating System Concepts – 8th Edition
14.26
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Consider the organization of a UNIX file as represented by
the inode. Assume there are 12 direct block pointers and
a singly, doubly and triply indirect pointer in each inode.
Further, assume that the system block size and the disk
sector size are both 8K. If the disk block pointer is 32
bits, with 8 bits to identify the physical disk and 24 bits to
identify the physical block, then:

What is the maximum file size supported by this system?

What is the maximum file system partition supported by this
system?

Assuming no information other than the file inode is already in
main memory, how many disk accesses are required to access
the byte in position 100K+10, where K=1024?
Operating System Concepts – 8th Edition
14.27
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A UNIX V7 directory entry
Operating System Concepts – 8th Edition
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Reminder: Disk organization in UNIX
Operating System Concepts – 8th Edition
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The steps in looking up /usr/ast/mbox
Operating System Concepts – 8th Edition
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Disk Space Management
(a) Storing the free list on a linked list
(b) A bit map
Operating System Concepts –
8th
Edition
14.31
Silberschatz, Galvin and Gagne ©2009