MODERN OPERATING SYSTEMS Third Edition ANDREW S. TANENBAUM Chapter 4 File Systems Tanenbaum, Modern...

MODERN OPERATING SYSTEMSThird Edition

ANDREW S. TANENBAUM

Chapter 4File Systems

Essential requirements for long-term information storage:

• It must be possible to store a very large amount of information.

• The information must survive the termination of the process using it.

• Multiple processes must be able to access the information concurrently.

File Systems (1)

Think of a disk as a linear sequence of fixed-size blocks and supporting reading and writing of blocks. Questions that quickly arise:

• How do you find information?• How do you keep one user from reading another’s data?• How do you know which blocks are free?

File Systems (2)

• Processes (threads), address spaces, files are the most important concepts in OS

• Files are logical units of information created by processes– Similar to kind of address space

• File system– Manage files: how they are structured, named,

accessed, used, protected, implemented, etc

File naming• Files are abstraction mechanism

– To store information on the disk and read it back– When a process creates a file, it gives the file a name;

and the file can be accessed by the name• Two-part file name

– File extension: indicating characteristics of file– In Unix, file extension is just convention; C compiler is

exception– In windows, file extensions specify which program

“owns” that extension; when double clicking, program assigned to it is launched

Figure 4-1. Some typical file extensions.

File Naming

Figure 4-2. Three kinds of files. (a) Byte sequence. (b) Record sequence. (c) Tree.

File Structure

File Types

• Regular Files:– ASCII files or binary files– ASCII consists of lines of text; can be displayed and printed– Binary, have some internal structure known to programs that

use them

• Directory– Files to keep track of files

• Character special files (a character device file)– Related to I/O and model serial I/O devices

• Block special files (a block device file)– Mainly to model disks

Figure 4-3. (a) An executable file. (b) An archive.

File Types

File Access• File descriptor

– A file descriptor is a small integer representing a kernel-managed object that a process may read from or write to

– Every process has a private space of file descriptors starting at 0

– By convention, 0 is standard input, 1 is standard output, and 2 is standard error

• System call: read() and write() to read from and write to filles named by file descriptors

Figure 4-4a. Some possible file attributes.

File Attributes

Struct stat

• Struct stat{ mode_t st_mode; // file type and mode (permission) ino_t st_ino; //inode number dev_t st_dev; //device number dev_t st_rdev; //device number (special) nlink_t st_nlink; //number of links uid_t st_uid; // user ID of owner

gid_t st_gid; // group ID of owneroff_t st_size; //size in bytes

time_t st_atime; //time of last access

……}

File attributes

• drwxr-xr-x 2 root root 4096 Sep 24 2008 Unit2• drwxr-xr-x 2 root root 4096 May 26 19:21 a• -rwxr-xr-x 1 root root 10930 Aug 5 22:49 a.out• -rwxrwx--T 1 root root 81 Aug 2 2008 a.txt• -rwxr-x--- 1 root root 81 May 26 19:20 b.txt• -rwx------ 1 root root 81 Jul 30 19:28 c.txt• -rwxr-xr-x 1 root root 11193 Jul 30 19:27 cp

File Owner

Group Owner

Everyone Else

Write Permission

Read Permissio

ExecutePermissio

File Owner

Group Owner

Everyone Else

UNIXUNIX

The most common system calls relating to files:

File Operations

• Append• Seek• Get Attributes• Set Attributes• Rename

• Create• Delete• Open • Close• Read• Write

Figure 4-5. A simple program to copy a file.

Example Program Using File System Calls (1)

Figure 4-5. A simple program to copy a file.

Example Program Using File System Calls (2)

Figure 4-6. A single-level directory system containing four files.

Hierarchical Directory Systems (1)

Single-level directory system: The simpliest

Figure 4-7. A hierarchical directory system.

Hierarchical Directory Systems (2)

Figure 4-8. A UNIX directory tree.

Path Names

Path names

• Absolute path name– Start from root and are unique

• Relative path name– Current working set: all path names not beginning

at the root directory are taken relative to the working directory

– If current working directory is /usr/ast:• then cp /usr/ast/mailbox /usr/ast/mailbox.bak• Is: cp mailbox mailbox.bak

Path names

• Special Entries:– “.” and “..”– Dot refers to the current directory; dotdot refers

to the parent directory (except root)– cp /usr/lib/dictionary .– cp /usr/lib/dictionary dictionary– Cp /usr/lib/dictionary /usr/ast/dictionary

System calls for managing directories:

Directory Operations

• Readdir• Rename• Link• Unlink

• Create• Delete• Opendir • Closedir

Directory operation

• Hard link– Linking allows a file to appear in more than one

directory; increments the counter in the file’s i-node

• Symbolic link– A name is created pointing to a tiny file naming

another file

File System Implementation

• Users:– How files are names, what operations are allowed

on them, what the directory tree looks like

• Implementors– How files and directories are stored, how disk

space is managed and how to make every thing work efficiently and reliably

File System Layout

• File system are stored on disks.• Most disks are divided up into several partitions• Sector 0 is called MBR (master boot record), to boot

the computer• BIOS reads in and executes MBR, MBR locates the

active partition, reads in the boot block, and execute• The boot block reads in the OS contained in the

partition

Figure 4-9. A possible file system layout.

File System Layout

Superblock: contains all the key parameters about a file system; read into memory the booted or the FS is used

Figure 4-10. (a) Contiguous allocation of disk space for 7 files.

(b) The state of the disk after files D and F have been removed.

Contiguous Allocation

• Advantage：– Simple to implement; to record the first block and

length– Easy to read; only one seek is needed

• Disadvantage:– Fragmentation

Figure 4-11. Storing a file as a linked list of disk blocks.

Linked List Allocation

Linked list allocation

• To keep each file as a linked list of disk blocks• Advantage

– Only internal fragmentation

• Disadvantage– Random access is difficult– Adding a pointer at the head of block; extra

overhead while copying

Figure 4-12. Linked list allocation using a file allocation table in main memory.

Linked List Allocation Using a Table in Memory

• FAT-File Allocation Table• Advantage

– Can take use of the whole block– Random access is easy– only to store the starting block number

• Disadvantage– To keep the entire table in memory– Can’t scale well

Figure 4-13. An example i-node.

I-nodes

i-nodes

• Advantage– i-node need only be in memory when the

corresponding file is open; file table grows linearly with the disk

• Disadvantage– Each i-node has fixed size

Figure 4-14. (a) A simple directory containing fixed-size entries with the disk addresses and attributes in the directory entry.

(b) A directory in which each entry just refers to an i-node.

Implementing Directories (1)

Figure 4-15. Two ways of handling long file names in a directory. (a) In-line. (b) In a heap.

Implementing Directories (2)

Figure 4-17. (a) Situation prior to linking. (b) After the link is created. (c) After the original owner removes the file.

Shared Files (2)

Figure 4-20. Percentage of files smaller than a given size

(in bytes).

Disk Space Management Block Size (1)

Figure 4-21. The solid curve (left-hand scale) gives the data rate of a disk. The dashed curve (right-hand scale) gives the disk

space efficiency. All files are 4 KB.

Disk Space Management Block Size (2)

Figure 4-22. (a) Storing the free list on a linked list. (b) A bitmap.

Keeping Track of Free Blocks (1)

Figure 4-23. (a) An almost-full block of pointers to free disk blocks in memory and three blocks of pointers on disk. (b) Result of freeing a three-block file. (c) An alternative strategy for handling the three free blocks. The shaded entries represent pointers to free disk blocks.

Keeping Track of Free Blocks (2)

Figure 4-31. The MS-DOS directory entry.

The MS-DOS File System (1)

FAT versions

• FAT comes up with 3 versions:– FAT 12– FAT 16– FAT 32– Differ in how many bits a disk address contains– Block size also varies: 512B, 1K, 2K,4K,…,32K

Figure 4-32. Maximum partition size for different block sizes. The empty boxes represent forbidden combinations.

The MS-DOS File System (2)

Figure 4-33. A UNIX V7 directory entry.

The UNIX V7 File System (1)

Figure 4-34. A UNIX i-node.

Figure 4-35. The steps in looking up /usr/ast/mbox.

exercises

• Consider a disk with 1 MB per track, a rotation time of 8.33 msec, and an average seek time of 5 msec. Then what is the time to read a block of k bytes?

exercise

• Consider the idea behind Fig.4-21, for a disk with a mean seek time of 8msec, a rotational rate of 15,000 rpm, and 264,144 bytes per track. What are the data rate for block sizes of 1KB, 2KB and 4KB, respectively?

Exercise

• How many disk operations are needed to fetch the i-node for the file /usr/ast/courses/os/handout.t? Asume that the i-node for the root directory is in memory, but nothing else along the path is in memory. Also assume that all directories fit in one disk block.

MODERN OPERATING SYSTEMS Third Edition ANDREW S. TANENBAUM Chapter 4 File Systems Tanenbaum, Modern...

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