1. Today topics: File System Implementation
Data Structures for files
Unix files, Inodes

2. Review: Representing an Open File (3)
file descriptor
table
system file
table
active inode
table 1 2 3 1 r 10
fdrw 1 rw 20 . .
fdr .
n–1
ref count
f pointer
access
inode
In this slide we see the effect of two opens of the same file within the same process.
fdrw = open("x", O_RDWR);
fdr = open("x", O_RDONLY);
write(fdrw, buf, 20);
read(fdr, buf2, 10);
disk
buffer cache
Copyright © 2002 Thomas W. Doeppner. All rights reserved.

3. Review: Representing an Open File (4)
file descriptor
table
system file
table
active inode
table 1 2 3
fdrw 2 rw 20 .
fdrw2 . .
n–1
ref count
f pointer
access
inode
The dup system call causes two file descriptors to refer to the same file table entry and hence share the offset.
fdrw = open("x", O_RDWR);
fdrw2 = dup(fdrw);
write(fdrw, buf, 20);
disk
buffer cache
Copyright © 2002 Thomas W. Doeppner. All rights reserved.

4. Review: Representing an Open File (5)
file descriptor
table
system file
table
active inode
table 1 2 3 2 r 10
fdrw 4 rw 20 .
fdrw2 .
fdr .
n–1
ref count
f pointer
access
inode
If our process executes a fork system call, creating a child process, the child is given a file-descriptor table that’s a copy of its parent’s. Of course, the reference counts on the system-file-table entries are increased appropriately.
fork( )
disk
buffer cache
Copyright © 2002 Thomas W. Doeppner. All rights reserved.

5. Review: I/O Redirection
% who > file &
if (fork( ) == 0) {
char *args[ ] = {"who", 0};
close(1);
open("file", O_WRONLY|O_TRUNC, 0666);
execv("who", args);
printf("you screwed up\n");
exit(1); }
This is an example of what a shell might do to handle I/O redirection: it first creates a new process in which to run a command (“who”, in this case). In the new process it closes file descriptor 1 (standard output—to which normal output is written). It then opens “file” (the arguments indicate that “file” is only to be written to, that any prior contents are erased, and that if “file” didn’t already exist, it will be created with read and write permission for all; assuming that file descriptor 0 is not available (it’s assigned to standard input), file descriptor 1 will be assigned to “file”. Assuming that execv succeeds, when “who” runs, its output is written to “file”.
Note that the parent process does not wait for its child to terminate; it goes on to execute further commands. (This behavior occurs because we’ve placed an “&” at the end of the command line.)
Note the args argument to execv: By convention, the first argument to each command is the name of the command (“who” in this case). To indicate that there are no further arguments, a zero is supplied.
Note that we aren’t checking for errors: this is only because doing so would cause the resulting code not to fit in the slide. You should always check for errors.

6. Today topics: File System Implementation
Data Structures for files
Unix files, Inodes

7. Unix File System – Inodes
Owner snt
Group cpre308
Type regular file
Perms rwxr-xr-x
Accessed oct pm
Modified ….
Inode modified …
Size bytes
Disk addresses
data structure on disk
one inode per file

8. Structure of a disk drive

9. File system layout A disk is divided into partitions
Each partition can have its own file system
Sector 0 of the disk is Master Boot Record (MBR)
Contain partition table
Starting and ending addresses of each partition
One partition is active
The first block called Boot Block
Contains a program to load operating system

10. Implementing files-goal
Disk = (long) sequence of blocks
Keep track of the blocks associated with a file

11. Methods Contiguous allocation Linked list allocation
Linked list allocation with a table in memory
I-nodes

12. Contiguous Allocation
All disk blocks of a file allocated sequentially
Advantages
(very) Fast read
Useful for read-only file systems (CD-ROM)
Keeping track of blocks of a file is easy
Problems
Fragmentation with deletes
File growth might be expensive X A B .
A expands X . B A

13. Sequential access is fast, random access is slow
Linked List of Blocks
Sequential access is fast, random access is slow

14. File Allocation Tables (FAT)
One entry per physical disk block;
FAT can be in main memory

15. Disk Map in Unix 1 2 3 4 5 6 7 8 9 10 11 12
The purpose of the disk-map portion of the inode is to represent where the blocks of a file are on disk. I.e., it maps block numbers relative to the beginning of a file into block numbers relative to the beginning of the file system. Each block is 1024 (1K) bytes long. (It was 512 bytes long in the original Unix file system.) The data structure allows fast access when a file is accessed sequentially, and, with the help of caching, reasonably fast access when the file is used for paging (and other “random” access).
The disk map consists of 13 pointers to disk blocks, the first 10 of which point to the first 10 blocks of the file. Thus the first 10Kb of a file are accessed directly. If the file is larger than 10Kb, then pointer number 10 points to a disk block called an indirect block. This block contains up to 256 (4-byte) pointers to data blocks (i.e., 256KB of data). If the file is bigger than this (256K +10K = 266K), then pointer number 11 points to a double indirect block containing 256 pointers to indirect blocks, each of which contains 256 pointers to data blocks (64MB of data). If the file is bigger than this (64MB + 256KB + 10KB), then pointer number 12 points to a triple indirect block containing up to 256 pointers to double indirect blocks, each of which contains up to 256 pointers pointing to single indirect blocks, each of which contains up to 256 pointers pointing to data blocks (potentially 16GB, although the real limit is 2GB, since the file size, a signed number of bytes, must fit in a 32-bit word).
This data structure allows the efficient representation of sparse files, i.e., files whose content is mainly zeros. Consider, for example, the effect of creating an empty file and then writing one byte at location 2,000,000,000. Only four disk blocks are allocated to represent this file: a triple indirect block, a double indirect block, a single indirect block, and a data block. All pointers in the disk map, except for the last one, are zero. All bytes up to the last one read as zero. This is because a zero pointer is treated as if it points to a block containing all zeros: a zero pointer to an indirect block is treated as if it pointed to an indirect block filled with zero pointers, each of which is treated as if it pointed to a data block filled with zeros. However, one must be careful about copying such a file, since commands such as cp and tar actually attempt to write all the zero blocks!
Copyright © 2002 Thomas W. Doeppner. All rights reserved.

16. Optimization for Sparse Files
1311
1423 12
Suppose a file was large, but mostly zeros
Could be produced using lseek and write

17. Additional Enhancements
Performance depends on: How many disk accesses are needed to read a file?
Store some data in the inode itself
Perhaps the whole file will fit in!
Need only 1 disk access for a small file
Increase block size

18. Implementing directories
Goal: which files are in a directory

19. Unix Directory (V7)
Directories are files whose data is a list of filenames & inodes
filename (14 bytes)
inode number (2 bytes) . 12 .. 14
etc
134
mail
346
crash 5
init
175
mount
586
Example inode
Owner snt
Group cpre308
Type regular file
Perms rwxr-xr-x
Accessed oct pm
Modified ….
Inode modified …
Size bytes
Disk addresses
Max filename size = 14 chars

20. The steps in looking up /usr/ast/mbox
The UNIX V7 File System
The steps in looking up /usr/ast/mbox