1. Operating System Concepts and Techniques Lecture 17
Information management-1*
Foundations and UNIX/Linux File System
M. Naghibzadeh
Reference
M. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011.
To order: or
* Information management is not usually covered in the first university course of operating systems

2. Storage media Disk pack: An structured hardware consisting of
Many plates, two surfaces per plate, many same capacity tracks on each surface, many same capacity sectors on each track
Disk model
Each partition
Partition Partition Partition k
Sector 1
Sector 1 Sector … … … Sector mn
...
Track Track m-1

3. File
An structure of vast amount of information to be stored on persistent media, could be
Collection of bytes
ASCII
Binary
Collection of records
Tree structure
File types
Regular: user files
Directories: meta data about files
Special Files: for example, I/O files for printer
Block special: files for tapes, disks

4. File Attributes/Data/operations/access
Each file consists of:
Attributes: owner, creator, password, Archive/system, Hidden, lock, temporary, random access, ASCII/Binary, time of last access, creation time, key length, key position, record length, max size, current size, time of last change etc.
Data
Valid operations are:
create, delete, open, close, read, write, append, seek, get attributes, set attributes, rename
File access:
Sequential, random

5. Storage allocation
Contiguous allocation: a file (or directory) is considered a chunk of data that must be stored in a contiguous data area of the storage device
Size of each file is a multiple of blocks
Block size is a power of two sectors
Advantages:
address calculation is simple
For sequential access prefetch is possible
Disadvantage
External fragmentation
May require frequent defragmentation
Disk attributes, metadata, …
File 1
File 2 …
File i
Unallocated area

6. Storage allocation...
Chained allocation: Data blocks of a file need not take a contiguous area, but they are chained together
Advantage
No external fragmentation
Disadvantages:
Address calculation is difficult
Random access is time consuming and needs excessive disk access
Disk attributes, metadata, …

7. Storage allocation...
Indexed allocation: any block of data can sit in any block frame of disk
Index table(s), will make note of which block is stored where, it uses disk block numbers instead of the physical address of the block; options are:
Only one table: like FAT
One table for every file or directory: like UNIX
Advantages:
No external fragmentation
index table is usually in the main memory thus rendering both sequential and random access to files and directories efficient

8. Other topics Free space management Bad block management
Chained free pieces
Bitmap
Index table
Bad block management
Substitute sectors
File of bad blocks

9. Directory tree/directory
Directory tree: the organization of files and directories, on a volume, in the form of a tree
Directory: a folder of files and subdirectories and their handles
Figure 1
Directory operations:
Create, delete, opendir, closedir, readdir, rename, link, unlink B C R S W X A D P Q L K
Directory
File

10. UNIX File System(USF) Structure of a partition is:
Superblock has attributes and metadata of the file system itself; size in blocks of the file system; size in blocks of the i-node list, number of free blocks and i-nodes, free block list
Indexed allocation is used to allocate available blocks to file and directory data. Every file or directory has its own index table
The index table is a special one, called index-node (or i-node for short)
The length of all i-nodes is the same, usually 64 bytes and the total number
If total number of i-nodes is say 1024, then the space requirement for this area is 64*1024 bytes or 64 kilo bytes
Boot block Superblock i-node list Data blocks

11. UNIX File System(USF)…
Root does not have a name, instead its i-node is the first i-node of the i-node list
At computer start/restart root directory becomes current directory
The last 4 rows of I-node needs special attention
File mode
Flags for file type, execution, owner permissions, Group permissions, and other permissions
Link count
Number of links to this i-node
OwnerID
ID of the owner
GroupID
ID of the group of the owner of this file
File size
File size in bytes
Last accessed
Time of last access
Last modified
Time of last modification
Inode modified
Time of last i-node modification
Direct addresses
Ten direct addresses
Single indirect addresses
Pointer to a block of addresses
Double indirect addresses
Triple indirect addresses
Some information of i-node

12. I-node In memory i-node is 64 bytes; the rest is on disk extensions
Ten block address for the file data (3 bytes each)
One single indirect
One double indirect
One triple indirect .
Data blocks
Table of pointers
File or
directory attributes
10 Direct pointers to
data blocks
Single indirect
Double indirect
Triple indirect

13. Largest file size Varies with: block sizes; assume 1K
Size of disk addresses used; assume 4 bytes
Direct addresses: total 10K
Single indirect addresses: total 256K
Double indirect addresses: total 256*256K
Triple indirect addresses: total 256*256*256K
Overall total:( * *256*256)K
Not too big, but a larger block size allows a larger file

14. (b) Directory C’s data of Figure 1 Root directory’s data of Figure 1
Directory data
2 bytes bytes
Format of directory data records
I-node number
File name 1 00 5 A 8 B 6 C 4 D 6 1 00 12 S 9 R
(b) Directory C’s data of Figure 1
Data block no 120
Root directory’s data of Figure 1
Data block no 100
Figure 2: The structure of directory data blocks

15. Looking for a file’s data
How can the file Q’s data storage location be found?
The operating system goes to current directory’s i-node and locates Q and its i-node number
Suppose file Q is not in the current directory but its absolute path is say /C/S/Q
The file system will access i-node number 1 and search for the root’s data location (data block no 100, Figure 2a)
It looks for C and its i-node number which is 6; using i-node number six it looks for directory C’s data block (say no. 120)
In this databblock (Figure 2b) the file system looks for S and its i-node number (say 12)
It continues this process until Q’s i-node number is found; using this i-node the file system searches for Q’s data storage blocks.

16. Advantage/Disadvantage
Advantages:
i-nodes can be shared while in main memory, hence file sharing
Fast processing
Disadvantage:
i-node updates are not instant, in memory i-node may be changed while on disk is something different

17. Summary
Caches and main memories lose their contents as soon as the computer is turned off
We must store persistent information on persistent devices
The well thought out organization of information in secondary storage, especially in light of its vastness, is extremely crucial
This responsibility lies with the file system
Many file systems have been developed to address this task
In this lecture, the design foundation of file systems was studied
The design details of a major file systems, the UNIX File System (UFS) was thoroughly investigated

18. Find out
The differences between Unix file system and Linux file system
All the information that is stored in a 64 byte i-node
Why the name of files and subdirectories of a directory should not be stored within the i-node of that directory and they have to be stored in data blocks?
The difference between a file system and a file
How can a file be organized to represent all bad blocks of a disk
What is the usage of a substitute sector

19. Any questions?