1. Operating Systems File systems
phones off (please)
CSCI2413 Lecture 8
Operating Systems
File systems

2. Lecture Outline File Principles File Allocation File Allocation Table
Contiguous
linked-list
Indexed
File Allocation Table
MS-DOS
NTFS
UNIX
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CSCI L8

3. Basic File Principles
To create a new file, a new directory entry is made in the logical file system
To use a file for I/O, the directory structure has to be searched to find its attributes and data blocks
at each read/write this would entail high overhead
A file is opened once at the start of use
the operating system copies the file information into a table in operating system memory
the file is then referred to by index into this table
in UNIX these indices are called file descriptors
in Windows / WinNT they are called file handles
© De Montfort University, 2004/5
CSCI L8

4. Contiguous Allocation
Each file is stored in a single group of adjacent blocks on the hard disk
simple to implement
excellent performance for file reads
The same principles apply as in the contiguous allocation of primary memory
similar allocation algorithms can be used
e.g. first-fit or best-fit
As with memory allocation, this scheme suffers from fragmentation problems
only worse, as de-fragmentation takes much longer!
© De Montfort University, 2004/5
CSCI L8

5. Non-Contiguous Allocation
Keep each file as a linked-list of disk blocks
the directory entry points to the first block number
each block then points to the next block in the list
directory entry
first
file
block 1
next
file
block 2
next
file
block 3
next
file
block 4
end
This overcomes the allocation problem, but
random file access is very slow
© De Montfort University, 2004/5
CSCI L8

6. Indexed Allocation
Keep a list of all blocks allocated to a file in a special index table
an array of disk block numbers that comprise the file
non-allocated blocks have an index of a special value
usually null (zero)
Non-contiguous storage allowed
random access is quick
However, a lot of space wasted
© De Montfort University, 2004/5
CSCI L8

7. Allocation Illustrations
contiguous
linked list
indexed
directory
name start size
file1.doc 0 2
file2.doc 5 5
file3.doc 16 8
directory
name start
file1.doc 0
file2.doc 1
file3.doc 2
directory
name index
file1.doc 0
file2.doc 8
file3.doc 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
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CSCI L8

8. Partition Tables
The very first sector on a hard drive is the primary boot sector which also contains the partition table
A partition is a collection of adjacent blocks on the hard drive that are grouped together
The partition table defines contiguously allocated blocks by start block and size
first partition is referred to as logical drive C:, D:, etc.
each partition has its own OS and so its own logical file system
e.g. MS-DOS/Win9x, Windows NT, Linux
© De Montfort University, 2004/5
CSCI L8

9. Directory Entries
MS-DOS/Win essentially uses the linked-list allocation method with one important refinement
the links are not held with the data blocks, but are kept in a special array, called the file allocation table (FAT)
blocks are referred to as clusters
A directory entry consists of
the file name, other information and the start block
the root directory is held at a fixed location on the hard disk and is of a fixed size (with 112 entries maximum)
file name 8 1
attribute flags
ext 3
reserved 10
time 2
date 2
start 2
size 4
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CSCI L8

10. Clusters CLUSTERS; Typical sizes of clusters:
1 sector is too small to adopt as the main unit of storage;
Sectors are grouped together into fixed size units called
CLUSTERS;
Typical sizes of clusters:
bytes ( 2K - 4 sectors)
bytes ( 4K - 8 sectors)
bytes (64K sectors)
© De Montfort University, 2004/5
CSCI L8

11. File Allocation - MS-DOS
Cluster
Next Cluster
FFFD 1
FFFF 2
0003 3
0004 4
0005 5
0006 6
0007 7 8
0000
FAT: a table of pointers to clusters:
The pointer points to the next cluster in which the file continues.
The cluster in which the end of file occurs has an entry of FFFF
An empty cluster is given an entry of 0000
Start of file
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CSCI L8

12. FAT System How many clusters can we have on a disk?
What is the max. amount of usable disk space?
The FAT table can be selected as 16 bit or 32 bit
16-bit FAT gives entries
the file system is compatible with MS-DOS
64K entries  32K clusters  2Gb maximum partition size
32-bit FAT gives 4  109 entries
extremely large maximum partition size!
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CSCI L8

13. NTFS Shortcomings of the (V)FAT structure is overcome by NTFS:
full access protection security
large capacity allocation scheme
internationalisation using long unicode file names
robust fault-tolerance with transaction logging
NTFS uses 64-bit disk addresses, supporting disk partitions of up to 264 bytes
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CSCI L8

14. NTFS Structures NTFS uses a complex index allocation scheme
each NTFS volume is organised as a linear sequence of blocks (clusters) – block size can be 512 bytes to 64 Kb
the principal component is the master file table (MFT)
MFT is a variable size file that holds records describing every file and directory in a volume
a mirror (copy) of the MFT is kept near the middle
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CSCI L8

15. File Space Allocation – Unix Systems
Unix systems have a pool of fixed size data structures called I-Nodes
When a file is created, it is given an i-node which summarises all information about the file.
Field Example data
Owner of file p
Group Id (group to which user belongs) students
File type regular
Permissions rwxr-xr-x
Last accessed 25 Nov
Last modified 20 Nov Last I-Node modification 20 Nov File Size bytes
No. of links (how many aliases) 2
Pointers to disk allocations 13
I-Node
Structure
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CSCI L8

16. File Space Allocation – Unix Systems
Pointers point to disk blocks allocated to the file.
Disk blocks are typically 512 bytes
Pointers may point directly to a block or else point to index blocks which contain another set of pointers
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CSCI L8

17. UNIX I-node The i-node is organised as 13 3-byte bytes addresses:
10 addresses point to the first 10 data blocks of the file.
The last 3 addresses point to block on disk that contains the next portion of the index, which contain pointers to succeeding blocks in the file.
© De Montfort University, 2004/5
CSCI L8

18. I-node Structure Data blocks file attributes block 1 block 2 block 3
single indirect
double indirect
triple indirect 11
266
Data blocks
65803
66058
131083
131338
huge!
267
522
65547
65802
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CSCI L8

19. Summary MS-DOS systems used the FAT method of managing files.
FAT-based systems had many deficiencies, most important of these was the lack of security features.
Early versions of Windows were merely interfaces to the basic MS-DOS OS using FAT.
NTFS replaced FAT in Windows NT (organisational use) and Windows95, Windows 2000, Windows XP (personal use).
NTFS has many advantages over FAT and is one of the key components of the Windows NT OS.
Unix file system uses three main tables: the file descriptor table, the open file description table and the i-node table. The i-node table is the most important of these, containing all the administrative information about a file and the location of its blocks.
Protection is based on controlling read, write and execute access for the owner, group and others.

20. Appendices ….
A file allocation table (FAT) is a table that an operating system maintains on a hard disk that provides a map of the clusters (the basic units of logical storage on a hard disk) that a file has been stored in.
When you write a new file to a hard disk, the file is stored in one or more clusters that are not necessarily next to each other; they may be rather widely scattered over the disk.
The operating system creates a FAT entry for the new file that records where each cluster is located and their sequential order. When you read a file, the operating system reassembles the file from clusters and places it as an entire file where you want to read it.
Virtual File Allocation Table (VFAT) is the part of the Windows 95 and later operating system that handles long file names, which otherwise could not be handled by the original file allocation table programming.

21. ….
NTFS (NT file system; sometimes New Technology File System) is the file system that the Windows NT operating system uses for storing and retrieving files on a hard disk. NTFS is the Windows NT equivalent of the Windows 95 file allocation table (FAT) and the OS/2 High Performance File System (HPFS). However, NTFS offers a number of improvements over FAT and HPFS in terms of performance, extendibility, and security.
Notable features of NTFS include:
Use of a B-tree directory scheme to keep track of file clusters
Information about a file's clusters and other data is stored with each cluster, not just a governing table (as FAT is)
Support for very large files (up to 2 to the 64th power or approximately 16 billion bytes in size)
An access control list (ACL) that lets a server administrator control who can access specific files
Integrated file compression
Support for names based on Unicode
Support for long file names
Data security on both removable and fixed disks

22. How NTFS Works
When a hard disk is formatted (initialized), it is divided into partitions or major divisions of the total physical hard disk space. Within each partition, the operating system keeps track of all the files that are stored by that operating system. Each file is actually stored on the hard disk in one or more clusters or disk spaces of a predefined uniform size. Using NTFS, the sizes of clusters range from 512 bytes to 64 kilobytes. Windows NT provides a recommended default cluster size for any given drive size. For example, for a 4 GB (gigabyte) drive, the default cluster size is 4 KB (kilobytes). Note that clusters are indivisible. Even the smallest file takes up one cluster and a 4.1 KB file takes up two clusters (or 8 KB) on a 4 KB cluster system.
The selection of the cluster size is a trade-off between efficient use of disk space and the number of disk accesses required to access a file. In general, using NTFS, the larger the hard disk the larger the default cluster size, since it's assumed that a system user will prefer to increase performance (fewer disk accesses) at the expense of some amount of space inefficiency.
When a file is created using NTFS, a record about the file is created in a special file, the Master File Table (MFT). The record is used to locate a file's possibly scattered clusters. NTFS tries to find contiguous storage space that will hold the entire file (all of its clusters).