1. File Systems

2. Linked block allocation
Each block contains a header with
Number of bytes in the block
Pointer to next block
Blocks need not be contiguous
Files can expand and contract
Seeks can be slow
First block …
Head: 417
...
Length
Length
Length
Byte 0
Byte 0
Byte 0
...
...
...
Byte 4095
Byte 4095
Byte 4095
Block 0
Block 1
Block N-1

3. DOS - Fat file system … … File Descriptor Disk Disk Block Disk Block43
254 …
107
Disk
Block
Disk
Block
Disk
Block
Logical view
File Descriptor 43
254 43 …
107
Disk
Block
Disk
Block
107
Disk
Block …
254
Physical view
The actual Table

4. Indexed Files Extract headers and put them in an index Simplify seeks
May link indices together (for large files)
Byte 0
...
Index block …
Head: 417
...
Byte 4095
Length
Block 0
Length
Byte 0
...
Byte 4095
Block 1
Byte 0
...
Length
Byte 4095

5. Index Structures Index Files Data Files max key Data block # 275 50
100
150
200 1 2 3 4
251
252
275
299
201
202
250
data
max key
IB 2
200
400
600 2 3 4
Data block 5
Data block 6
300
350
Index block1
250
300
350 5 6 7
IB 3
Data block 7

6. *X file structure inode Data Data Index Data Index Data Index Data
mode
owner …
Direct block 0
Direct block 1
Single indirect
Double indirect
Triple indirect
Data
Data
Index
Data
Index
Data
Index
Data

7. Indexed Systems this slide needs work
Level
Number of Blocks
Number of Bytes
Direct
N (10)
10K
Single indirect
256
256*1K/block=256K
Double indirect
256  256 = 65K blocks
65K blocks * 1K/block=65M bytes
Triple indirect
256  65K = 64M blocks
>16G bytes (64blocks*256K/block=16GB)
Blocksize=1K = 1024 bytes
1024/4=256 (size of a pointer)

8. *X or Win* mount Joe DJF mount DJF at any / / bin usr etc any bin usr
one
two
abc
cde
xyz
DJF /
more
abc
cde
xyz
mount DJF at any
more