3. Objectives
To explain the function of file systems
To describe the interfaces to file systems
To discuss file-system design tradeoffs, including access
methods, file sharing, file locking, and directory structures
To explore file-system protection
4. 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
5. File Operations
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
6. 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 processes closes it
Disk location of the file: cache of data access
information
Access rights: per-process access mode information
8. 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
10. Directory Structure
A collection of nodes containing information about all files.
F 1 F 2
F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on disk
11. Operations Performed on
Directory
List directory contents
Search for a file
Create a file
Delete a file
Rename a file
Traverse the file system
12. Single-Level Directory
A single directory for all user called the root
directory
Pros: Simple, easy to quickly locate files
Cons: inconvenient naming (uniqueness), no
grouping
13. Two-Level Directory
Separate directory for each user
Introduces the notion of a path name
Can have the same file name for different user
Efficient searching
No grouping capability
15. Path Names
To access a file, the user should either:
Go to the directory where file resides, or Specify the
path where the file is
Path names are either absolute or relative
Absolute: path of file from the root directory
Relative: path from the current working directory
Most OSes have two special entries in each directory:
“.” for current directory and “..” for parent
17. Protection
File owner/creator should be able to control:
what can be done
by whom
Types of access
Read
Write
Execute
Append
Delete
List
18. Categories of Users
Individual user
Log in establishes a user-id
Might be just local on the computer or could be
through interaction with a network service
Groups to which the user belongs
For example, “nahum” is in “w4118”
Again could just be automatic or could involve
talking to a service that might assign, say, a
temporary cryptographic key
19. UNIX Access Rights
Mode of access: read, write, execute
Three classes of users
RWX
a) owner access 7 1 1 1
RWX
b) group access 6 1 1 0
RWX
c) public access 1 0 0 1
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 group public
chmod 761 game
20. Issues with Access Rights
Just a single owner, a single group and the public
Pro: Compact enough to fit in just a few bytes
Con: Not very expressive
Access Control List: This is a per-file list that tells
who can access that file
Pro: Highly expressive
Con: Harder to represent in a compact way
21. Disk Scheduling
The operating system is responsible for using
hardware efficiently — for the disk drives, this means
having a fast access time and disk bandwidth.
Access time has two major components
Seek time is the time for the disk are to move the
heads to the cylinder containing the desired sector.
Rotational latency is the additional time waiting
for the disk to rotate the desired sector to the disk
head.
22. Disk Scheduling
Minimize seek time
Seek time seek distance
Disk bandwidth is the total number of bytes
transferred, divided by the total time between the
first request for service and the completion of the last
transfer.
23. Disk Scheduling
Several algorithms exist to schedule the servicing of
disk I/O requests.
We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
25. SSTF Disk Scheduling
Selects the request with the minimum seek time from
the current head position.
SSTF scheduling is a form of SJF scheduling; may
cause starvation of some requests.
Illustration shows total head movement of 236
cylinders.
27. SCAN Disk Scheduling
The disk arm starts at one end of the disk, and moves
toward the other end, servicing requests until it gets
to the other end of the disk, where the head
movement is reversed and servicing continues.
Sometimes called the elevator algorithm.
Illustration shows total head movement of 208
cylinders.
29. C-SCAN Disk Scheduling
Provides a more uniform wait time than SCAN.
The head moves from one end of the disk to the
other. servicing requests as it goes. When it reaches
the other end, however, it immediately returns to the
beginning of the disk, without servicing any requests
on the return trip.
Treats the cylinders as a circular list that wraps
around from the last cylinder to the first one.
31. LOOK and C-LOOK Disk
Scheduling
In LOOK and C-LOOK the arm goes as far as the final
request in each direction then reverses direction
immediately without first going all the way to the end
of the disk.
Versions of SCAN and C-SCAN that follow this
pattern are called LOOK and C-LOOK.
35. Contiguous Allocation
Each file occupies a set of contiguous blocks on the
disk
Pros:
Simple – only starting location (block #) and
length (number of blocks) are required
Random access
Cons:
Wasteful of space (dynamic storage-allocation
problem)
External fragmentation: may need to compact
space
Files cannot grow
37. Contiguous Allocation
Some newer file systems (i.e. high-performance
Veritas File System) use a modified contiguous
allocation scheme
Extent-based file systems allocate disk blocks in
extents
An extent is a contiguous chunk of blocks (similar to
clusters)
Extents are allocated when the file grows
Extents are linked
A file consists of one or more extents
Extent size can be set by owner of file
38. Linked Allocation
Each file is a linked list of disk blocks
Blocks may be scattered anywhere on the disk
Pros:
Simple – need only starting address
Free-space management system – no waste of
space
Cons:
No efficient random access
40. File Allocation Table
Variation on linked list allocation.
FAT is located in contiguous space on disk
Each entry corresponds to disk block number
Each entry contains a pointer to the next block or 0
Used by MS-DOS and OS/2
42. Indexed Allocation
Brings all pointers together into the index block
Pros:
Efficient random access
Dynamic access without external fragmentation
Cons:
Index table storage overhead
44. Indexed Allocation
Linked Scheme: If the size of index block is too
large link several index blocks together.
Last pointer in the block points next index block.
Nil indicates the end of index file.
45. Indexed Allocation
Multi level Index: First level index block points to
second level index blocks which point to file blocks.
outer-index
index table file
46. Indexed Allocation
Combined Scheme: first n-3 pointers point to data
blocks, last three are multilevel indexes.
First block points to single level index, second block
points to second level. Third block points to third
level.
