1. Operating
Systems
File Systems
CNS 3060

2. Topics Files Directories File system implementation
Operating
Systems
Topics
Files
Directories
File system implementation
Example file systems

3. Long-term Information Storage
Operating
Systems
Long-term Information Storage
Must store large amounts of data
Information stored must survive the termination of the process using it
Multiple processes must be able to
access the information concurrently

4. File Structure Three kinds of files
Operating
Systems
Three kinds of files
byte sequence (O/S doesn’t care what’s inside)
record sequence (O/S understands record structure)
tree (Database systems)

5. Sequential Access o read all bytes/records from the beginning
Operating
Systems
Sequential Access
o read all bytes/records from the beginning
o cannot jump around, could rewind or back up.
o convenient when medium was mag tape

6. Random Access o bytes/records read in any order – disk drives
Operating
Systems
Random Access
o bytes/records read in any order – disk drives
o essential for data base systems
o read can be …
- move file marker (seek), then read or …
- read and then move file marker

7. Typical File Attributes
Operating
Systems

8. 1. Create 2. Delete 3. Open 4. Close 5. Read 6. Write
Operating
Systems
File Operations
1. Create
2. Delete
3. Open
4. Close
5. Read
6. Write
Append
Seek
Get attributes
Set Attributes
Rename

9. Operating
Systems
Memory Mapped Files
In some cases, it is convenient to map a file into the address space of a running process. File access is then done by normal reads and writes of memory. The result is much faster and to some, much easier than actual writing to the file.
Memory mapping is done by changing the system’s internal tables so that the file becomes backing store (ala paging) for the memory region into which the file is mapped.

10. Hierarchical Directory Systems
Operating
Systems
Root A B C A B B C C B B C C C C

11. Path Names
Operating
Systems
A UNIX
directory tree

12. 1. Create 2. Delete 3. Opendir 4. Closedir
Operating
Systems
Directory Operations
1. Create
2. Delete
3. Opendir
4. Closedir
5. Readdir
6. Rename
7. Link
8. Unlink

13. A File System Implementation
Operating
Systems
sector 0
1 partition is marked as active
OS Loader
A possible file system layout
When the system is started, the BIOS reads in and executes the Master Boot Record (MBR). The MBR locates the active partition and finds it’s boot block. The boot block loads the O/S.

14. The superblock contains information
about the file system itself, for example,
how big is the file system
super
block
i-node table
data

15. This is an array of i-node structures. An i-node
is identified by it’s position in the array.
super
block
i-node table
data

16. I-Nodes
Operating
Systems
I-nodes are fixed in size.
When a file is opened, it’s i-node is loaded from disk into memory. Thus only a small amount of memory is required, and only while the file is opened.

17. All directories and files are stored in the data area
of the file system. Everything in the file system is
stored in “blocks ”. Blocks are fixed in size and represent
the smallest unit of storage in the file system.
super
block
i-node table
data

18. Creating a new file involves the following operations:
1. The O/S finds an unused i-node 47
super
block
i-node table
data

19. Creating a new file involves the following operations:
1. The kernel finds an unused i-node
2. The kernel stores file attributes in the i-node 47
super
block
i-node table
data
attributes

20. Creating a new file involves the following operations:
1. The kernel finds an unused i-node
2. The kernel stores file attributes in the i-node
3. The kernel find free blocks and stores the file data 47
super
block
i-node table
data
|||||||||
|||||||||
|||||||||
200
635
821
attributes

21. Creating a new file involves the following operations:
1. The kernel finds an unused i-node
2. The kernel stores file attributes in the i-node
3. The kernel find free blocks and stores the file data
4. The kernel stores the block number in the i-node 47
super
block
i-node table
data
|||||||||
|||||||||
|||||||||
200
635
821
attributes
200
635
821

22. Creating a new file involves the following operations:
1. The kernel finds an unused i-node
2. The kernel stores file attributes in the i-node
3. The kernel find free blocks and stores the file data
4. The kernel stores the block number in the i-node
5. The kernel adds an entry to the directory 47
super
block
i-node table
data
|||||||||
|||||||||
|||||||||
200
635
821
attributes
200
635 47
myFile.txt
821

23. Terminology Operating Systems Sector Block or cluster Track
Internal Fragmentation:
Occurs when all of a block is
not used by a file.
A block is the minimum
unit of storage for data.
Block sizes are defined
by the O/S and the
file system.
External Fragmentation:
Occurs when blocks used
to store a file are not
contiguous.
Read/write
head

24. Problem Operating Systems Given that you need n blocks on the disk
Sector
Operating
Systems
Block or cluster
Track 1 4
A block is the minimum
unit of storage for data.
Block sizes are defined
by the O/S and the
file system. 2 3
Read/write
head
Given that you need n blocks on the disk
to hold the contents of a file, how do you
allocate those blocks to the application?

25. Contiguous File Allocation
Operating
Systems
The simplest file allocation scheme is to take blocks
sequentially from the disk, as they are needed for each
file. This has two major advantages:
o It is simple to implement. You only need to keep track of
the starting block and the number of blocks in the file.
o It is very efficient. Only one seek is required to read in
the entire file. (a seek is the operation that moves the
read/write head over the correct track.)

26. Problem Operating Systems Sector Block or cluster Track Read/write1 2 4
Read/write
head

27. Contiguous File Allocation
Operating
Systems
Create a file of 3 blocks

28. Contiguous File Allocation
Operating
Systems
Create a file of 3 blocks
Create a file of 5 blocks

29. Contiguous File Allocation
Operating
Systems
Create a file of 3 blocks
Create a file of 5 blocks
Create a file of 4 blocks

30. Contiguous File Allocation
Operating
Systems
Create a file of 3 blocks
Create a file of 5 blocks
Create a file of 4 blocks
Create a file of 6 blocks

31. Contiguous File Allocation
Operating
Systems
What’s the problem with this design?
External Disk Fragmentation.
you have to have a file
that is 5 blocks or less
to fit here!
Create a file of 3 blocks
Create a file of 5 blocks
Create a file of 4 blocks
Create a file of 6 blocks
Now Delete the 2nd file

32. Linked List Allocation
Operating
Systems
+ Every block on disk can be used. Disk blocks can be anywhere.
+ The directory only need store the address of the first block of the file.
- Each block sacrifices the space required to store the pointer
- Random access of blocks in the file is slow.

33. what if you want to randomly
Random Access
Operating
Systems 1 4 2 3
what if you want to randomly
access this block?

34. The directory only contains the address of the first block. Operating
So ... you have to access this block,
because it contains the pointer
to the next block.
The directory only contains the
address of the first block.
Operating
Systems 1 4 2 3

35. Operating Systems Now you need to access this block1 4 2 3
Now you need to access this block
to get the location of the 3rd block
. . . but this will more than likely involve
a disk seek, i.e. move the disk head

36. A File Allocation Table (fat)
Operating
Systems
The fat table usually resides in a fixed location
at the beginning of on the disk.
No space is taken up in the file for pointers.

37. Why cache the fat? Operating Systems move the disk head to1 4
fat
move the disk head to
read the first entry in
the fat 2 3

38. Why cache the fat? Operating Systems now move the disk head to1 4
fat 2 3
now move the disk head to
read in the first block of
the file.

39. to read the next entry in
Why cache the fat?
Operating
Systems 1 4
fat
move the disk head back
to read the next entry in
the fat 2 3

40. Why cache the fat? Operating Systems Move the disk head to1 4
fat
Move the disk head to
read in the next block
in the file 2 3

41. When the fat is in cache Operating Systems
+ Random access is easier – the chain is entirely in memory
- The biggest disadvantage is that the FAT resides in memory
assume a 20GB disk with 1024KB block-size. The FAT needs
20 million entries (60-80MB)

42. Unix uses i-nodes!

43. Directory Implementations
Operating
Systems
mail
attributes
disk address
games
attributes
disk address
homework
attributes
disk address
music
attributes
disk address
photos
attributes
disk address
A simple Directory
* File attributes stored in the
directory
* Disk address stored in the
directory (first block)
* Fixed size entries (so fixed
length file names)
(MS/DOS & Windows3.x)

44. Directory Implementations
Operating
Systems
mail
address of i-node
games
address of i-node
homework
address of i-node
music
address of i-node
photos
address of i-node
Each directory entry points
to an i-node. File attributes
are stored in the i-node.
(Unix)

45. Handling Long File Names
Operating
Systems
Fragmentation Issues
(take a directory entry out)
Page Faults may occur
(directory spans multiple pages)

46. Handling Long File Names
Operating
Systems

47. Unix allows different processes to share files …
File Sharing
Unix allows different processes to share files …
The Process Table:
every process has an entry in the process table
includes all open file descriptors owned by the process
- file descriptor flags
- a pointer into the file table
process table entry
fd flags ptr
fd 0:
fd 1:
fd 2:
...

48. The File Table (per process) table of all open files
- status flags for the file (read, write, append, etc)
- the current file offset
- a pointer to the i-node for this file
file table entry
process table entry
file status flags
current file offset
i-node pointer
fd flags ptr
fd 0:
fd 1:
fd 2:
...

49. read from disk when the file is opened
The i-node Table
one for each open file
read from disk when the file is opened
includes a pointer to the file’s i-node
- file permissions
- file owner
- file size
- device file is physically located on
- pointers to the actual file blocks on disk
- etc
file table entry
process table entry
i-node
file status flags
current file offset
i-node pointer
permissions
user & group ids
File size
Time stamps
. . .
Pointer to first disk block
fd flags ptr
fd 0:
fd 1:
fd 2:
...

50. A Single Process With Two Open Files
i-node
permissions
user & group ids
File size
Time stamps
. . .
Pointer to first disk block
process table entry
file table
file status flags
current file offset
i-node pointer
fd flags ptr
fd 0:
fd 1:
fd 2:
...
file status flags
current file offset
i-node pointer
i-node
permissions
user & group ids
File size
Time stamps
. . .
Pointer to first disk block

51. Two Processes Sharing the Same File
process table entry
i-node
file table
file status flags
current file offset
v-node pointer
permissions
user & group ids
File size
Time stamps
. . .
Pointer to first disk block
fd flags ptr
fd 0:
fd 1:
fd 2:
...
file table
process table entry
file status flags
current file offset
v-node pointer
fd flags ptr
fd 0:
fd 1:
fd 2:
...
Note that as each process writes to the file, the file offset is
updated in the file table for that process, to reflect the number
of bytes written. If this causes the file offset to exceed the file
size, the file size is updated in the files i-node.

52. Sharing Files Operating Systems
In the simple directory case, both directories
must contain the block addresses for the file.
If user B adds to the file, the appended disk
blocks will not show up in C’s directory.
This file appears in User B’s directory as
well as in User C’s directory.
This problem is solved using i-nodes, since
each directory entry only need point to the
i-node.

53. Operating
Systems
i-node
file
data
B’s directory
C ’s directory

54. Deleting a Shared File User C creates a file Operating Systems
User C’s
directory
User C creates a file
i-node
owner = C
count = 1
i-node

55. Deleting a Shared File User B links to the shared file.
Operating
Systems
User C’s
directory
User B’s
directory
User B links to
the shared file.
i-node
What do you do when
User C deletes the file?
You can’t delete the file,
Because the B’s Directory
will point to an invalid i-node.
owner = C
count = 2
i-node

56. Deleting a Shared File User B links to the shared file.
Operating
Systems
User C’s
directory
User B’s
directory
User B links to
the shared file.
i-node
What do you do when
user C deletes the file?
You can’t delete the file,
because the B’s Directory
will point to an invalid i-node.
owner = C
count = 1
i-node
All you can do is delete C’s directory
Entry and leave the file. This works, but may cause accounting problems.

57. Using Symbolic Links Operating Systems User C’s User B’s directory
Using what is called a symbolic link,
B links to one of C’s files by creating a
new file type called a Link. The Link file
just contains the path name of the file
it links to.
type = file
i-node
type = link
i-node
file
data
path
file

58. Using Symbolic Links Operating Systems User C’s User B’s directory
Now, when C deletes the file,
the i-node pointed to by B’s
directory is valid. However,
attempts to access the file
will fail.
type = link
i-node
???
path
file

59. Disk Space Management Block Size
Operating
Systems
As we have seen, disk space is usually managed in fixed size blocks. The question arises, how big should the blocks be?
If the block is too large, then a lot of space is wasted.
If the block size is too small, then performance suffers
because reading each block requires a rotational delay
and a seek.

60. Determining Optimal Block Size
for 2KB files
data rate is almost completely
dominated by seek time and
rotational delay of the disk.
wasted
space
Block Size
Dark line (left hand scale) gives the data rate of a disk
Dotted line (right hand scale) gives disk space efficiency
Note that disk space utilization and data rate are in direct conflict!

61. Managing Free Disk Blocks
Operating
Systems
bit map
Linked List
n bytes to hold a disk block number
1 bit per block
Fixed size

62. The MS-DOS File System was patterned after
Operating
Systems
The MS-DOS File System was patterned after
the CP/M File System
Directory entry – fixed 32 byte length
bytes 8 3 1 10
File Name
Ext
Reserved
Time
Date
Size
First block
number in FAT
Attribute
read-only
archive
hidden
system file
Pinned to 1980
Accurate to within 2 seconds

63. FAT-12
Operating
Systems
MS-DOS defines a block (called a cluster by Microsoft) as some multiple of 512 bytes.
FAT-12 blocks were 512 bytes. Each cluster is represented in the File Allocation table by a 12 bit number. Thus, the maximum partition size was about 2MB and the File location
Table contained byte entries.

64. Fat-16
Operating
Systems
As drives got bigger, the File Allocation Table structure had to change to accommodate the bigger sizes. In a FAT-16 table, each cluster was now represented by a full 16 bit entry in the file allocation table. Additional block sizes of 8 KB, 16KB, and 32 KB supported 2 GB partitions.
But what’s the problem with bigger block sizes?

65. Fat-32 FAT-32 was introduced in the second release of Windows 98.
FAT-32 represents each cluster in the file table with a 28-bit
entry. The maximum partition size for a FAT-32 system is
2 Terabytes. A big advantage of FAT-32 is that for an
equivalent partition size, FAT-32 blocks can be much smaller
than FAT-16 blocks. Thus, the file system is more efficient.
The downside is that the File Allocation Table is much bigger.
For a 4KB block and a 2GB partition, the FAT takes up
2 MB of RAM.

66. The Windows 98 File System
NT bit (for compatibility)
Adds time accuracy to creation date/time - within 10ms
Last access time
bytes 8 3 1 10
File Name
Ext
Time
Date
Size
Lower 16 bits of
first block number
Attribute
read-only
archive
hidden
system file
Last write
Upper 16 bits of
first block number
Creation
date/time

67. Long File Names Windows 98 provides 2 file names for each file.
Operating
Systems
Windows 98 provides 2 file names for each file.
* The standard DOS file name
* A long file name
The algorithm for creating an file name
is to truncate the file name to 6 characters and
add ~1 to the name ( or ~2 if the name already exists).

68. Operating
Systems
Each long file name is stored in the directory, in front of the
normal directory entry for the file. Up to 13 Unicode characters
are stored in the following format. Multiple entries are used to
Provide storage for the entire long file name.
5 characters
6 characters
2 chars
Attribute byte
0x0F
Sequence
checksum

69. . . . The Big Long Name Operating Systems 65 a m e A C 1 T h e B A CC 1
T h e B A C
i g L o n
g N
Creation
Time
T h e B I g ~ 1 A NT S
. . .
low
The Big Long Name

70. Windows NT File System

71. NTFS does not use sectors. Instead
it uses a cluster, which is some number
(power of 2 ) sectors. This makes the
file system independent of sector size.

72. free space bitmap
boot
sector
system
files
master file table
data
MFT Records
Record
Header
attribute-value pairs
Attribute
header
data

73. Attribute
header
data
one of:
standard info
file name
attribute list (location of additional mft records, if needed)
volume name
data
. . .