1. Chapter 12: File-System Implementation CSS503 Systems Programming
Prof. Munehiro Fukuda
Computing & Software Systems University of Washington Bothell
CSS503
Chapter 12: File-System Implementation

2. Chapter 12: File-System Implementation
File System Layers
Applications
Access files through open, read, write, lseek, and close
Translate a file name into a file number an a file handle
Mange directory
Protect files
Manage each file in an FCB: file control block (inode in Unix, master file table entry in NTFS)
Find physical block# through FCB
Mange free free blocks
Translate “retrieve block #x” in drive X, cylinder Y, track 2, and sector 10
Buffer/cash data from disk to I/O buffers.
Access disks with drive X, cylinder Y, track 2, and sector 10
Logical file system
File-organization module
Basic file system
I/O control
Devices
CSS503
Chapter 12: File-System Implementation

3. Disk Structure and Directories
Sector 0
Partition table <bootable/non-bootable, sectors, cylinders>
Layout of the file system: file system size, #free blocks
File control blocks (inodes)
Files and directories:
<inode, filename>
<123, “.”>
<567, “data.txt”>
MBR: Master Boot Record
Partition Table
OS Boot Block
Super Block
File Space Management
inodes
Data Blocks
dir
file
Another Partition
CSS503
Chapter 12: File-System Implementation

4. A Typical File Control Block
Unix: iNode
NTFS: Master file table entry
File permission
File dates (create, access, write)
File owner, group, access contrl list
File size
File data block or pointers to file data blocks
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Chapter 12: File-System Implementation 3

5. Contiguous Allocation
directory
Merits
Good performance (minimal seek time)
Example
IBM VM/CMS
Problems
External fragmentation
Determining the file space upon its creation
(Can we predict the size before a file is written?)
File
Start
Length
count 2 tr 14 3
mail 19 6
list 28 f 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
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Chapter 12: File-System Implementation 4

6. Chapter 12: File-System Implementation
Linked Allocation
Merits
Need only starting block
No external fragmentation
Problems
Sequential access
Link information occupying a portion of block
File not recovered if its link is broken
CSS503
Chapter 12: File-System Implementation

7. File Allocation Table (FAT)
test
217
name
start block
directory entry
FAT has an entry for each disk block.
FAT entries rather than blocks themselves are linked.
Example:
MS-DOS and OS/2
Merit:
Save disk block space
Faster random accesses
Demerit:
A significant number of disk head seeks
339
618
EOF
217
#blocks -1
FAT
217
339
618
CSS503
Chapter 12: File-System Implementation

8. File Allocation Table (FAT)
FAT entry size: 12bits
#FAT entries: 4K
Disk block (cluster) size: 32K (depends on each system)
Total disk size: 128M
FAT16
FAT entry size: 16bits
#FAT entries: 64K
Disk block size: 32K
Total disk size: 2G
FAT32
FAT entry size: 32bits, of which 28bits are used to hold blocks
#FAT entries: 256M
Total disk size: 8T, (but limited to 2T due to the use of sector counts with the 32bit entry.)
CSS503
Chapter 12: File-System Implementation

9. Chapter 12: File-System Implementation
Indexed Allocation
Similar to the paging scheme
Merit:
Directory access
Demerits:
Internal fragmentation
Uncertainty in the index block size
Too small: cannot hold a large file
Too large: waste disk space
CSS503
Chapter 12: File-System Implementation

10. NTFS MFT (Master File Table):
An array of records, each holding the attributes for a different file
Sequence
File Number 63 47
File reference
MFT
File record: name, security, and data
For consistency
check
MFT entry - 1
Run(extent)
Run(extent)
Disk
Cluster numbers
Cluster numbers
CSS503
Chapter 12: File-System Implementation

11. NTFS Continued Directory:
Includes all the file references belonging to the directory in its runs
MFT
Directory “\”
File record: name, security, and data
Run(extent)
Run(extent)
Cluster numbers
Cluster numbers
file1 file2 file3 file4
file5 file6 file7 file8
CSS503
Chapter 12: File-System Implementation

12. Linked Scheme in Index Allocation
Advantage:
Adjustable to any size of files
Disadvantages:
Slower random accesses for larger files 4 5 6 . 28 27 29 30
CSS503
Chapter 12: File-System Implementation

13. Multilevel Index in Indexed Allocation
outer-index
Advantage:
Adjustable to any size of files
Disadvantages:
Multiple table accesses
index table
file
CSS503
Chapter 12: File-System Implementation

14. Combined Scheme: UNIX (4K bytes per block)
Inode
File information
The first 12 pointers point directly to data blocks
The 13th pointer points to an index block
The 14th pointer points to a block containing the addresses of index blocks
The 15th pointer points to a triple index block.
CSS503
Chapter 12: File-System Implementation

15. Unix File System Structure
Process
int fd = open(“fileA”, flags);
read(fd, …);
stdin
stdout
stderr 1 2 3
User file
Descriptor table
PCB
Inode:
length
count 1
direct[12]
indirect[3]
struct file:
count 1
inode
Disk
File Structure Table
Inode table
CSS503
Chapter 12: File-System Implementation

16. Chapter 12: File-System Implementation
Free Space Management
Bit vector (n blocks)
Linked free space list
Easy to find contiguous space
Pros
No waste of space
Waste of memory space
Cons
Difficult to find contiguous space 1 2
n-1
Free-space list head … 1 2 3 4 5 6 7
0  block[i] free
1  block[i] occupied 8 9 10 11
bit[i] =
 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
CSS503
Chapter 12: File-System Implementation

17. Disk Cache Utilizing paging Double-caching problem
Memory-Mapped I/O
I/O Using read( ) and write( )
Page Cache
Utilizing paging
File System
Double-caching problem
Buffer Cache
Improving performance
bread(dev, blkno, size), bwrite( bufp)
Block I/O Routine
Disks
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18. Page versus Buffer Cache
Regular file I/O with read( ) and write( )
A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O to avoid double caching
Memory-Mapped I/O
File System
File Meta Data: inodes, dentries (directories)
Page Cache
inode->i_pages
page 0
page 1
page n
in memory
Buffer Cache
buffer
hdr
page
dscptr
physical page
Buffer Page
buffer
hdr
page
dscptr
physical page
Buffer Page
bread(dev, blkno, size), bwrite(bufp)
block_read_full_page( page, get_block( ) )
Block I/O Routine
Disks
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19. Programming Assignment 3 Standard I/O Library
Standard I/O Library itself has a buffer to reduce # read and write system calls.
#include <stdio.h>
Int main( ) {
FILE *f = fopen( “myFile”, “r” );
char buf[10];
fgets( buf, sizeof( buf ), f );
printf( “%s¥n”, buf );
return 0; }
user
mode
Standard C Library
stdio buffer
System Call Interface: sd = open( “myFile”, O_RDONLY ), read( fd, buf, sizeof( buf ), write( 1, buf, sizeof( buf ) )
kernel
mode
CSS503
Chapter 12: File-System Implementation

20. Programming Assignment 3 Standard I/O Library
Since Unix does not differentiate between text and binary, ‘b’ has no effect.
fopen type
Unix open mode
r or rb
O_RDONLY
w or wb
O_WRONLY | O_CREAT | O_TRUNC
a or ab
O_WRONLY | O_CREAT | O_APPEND
r+ or rb+ or r+b
O_RDWR
w+ or wb+ or w+b
O_RDWR | O_CREAT | O_TRUNC
a+ or ab+ or a+b
O_RDWR | O_CREAT | O_APPEND
CSS503
Chapter 12: File-System Implementation

21. Chapter 12: File-System Implementation
Discussions
In which table should we implement the file seek pointer, a user file descriptor table, a file structure table, or an inode table?
Why? Consider the reason as focusing on the situation where a parent and a child process shares the same file at the same time when another independent process also reads the same file.
What is the similarity and difference in file allocation between Unix and Windows NTFS
CSS503
Chapter 12: File-System Implementation