Why files? 1. Storing a large amount of data 2. Long-term data retention 3. Access to the various processes in parallel

Transcription

1 1 File System

2 Why files? 1. Storing a large amount of data 2. Long-term data retention 3. Access to the various processes in parallel 2

3 Basic Terms File Structures Field basic unit of data. Contains single value and the name of the field. Record collection of related fields that can be treated as a unit by some application program. File collection of similar records. DataBase collection of related data and the relationships between them. 3

4 File Structures 1. Sequence of bytes. Maximum flexibility 2. Sequence of records. Working in parallel on different records 3. Tree. A quick search of records 4

5 5 File Structures

6 Sequential Files And Indexed Sequential Files 6

7 B Trees B-tree is a n-ary tree structure. B-trees of order d has the following properties: 1. Every node has at most 2d 1 keys and 2d children or, equivalently, 2d pointers. 2. Every node, except for the root, has at least d 1 keys and d pointers. 3. The root has at least 1 key and 2 children. 4. All leaves appear on the same level and contain no information. 5. A non-leaf node with k pointers contains k 1 keys. 7

8 8 B Trees node structures

9 B-tree insertion example B-tree of order 3 * 3 * 9 * 13 * a b d c e * 1 * 2 * * 4 * 5 * 7 * * 10 * 12 * * 21 * 9

10 B-tree insertion example Insert 8: * 9 * a f g * 3 * 7 * * 13 * b d h c e * 1 * 2 * * 4 * 5 * * 8 * * 10 * 12 * * 21 * 10

11 B-tree deletion example Delete 2, 21 * 9 * a f g * 3 * 7 * * 12 * b d h c e * 1 * * 4 * 5 * * 8 * * 10 * * 13 * 11

12 B-tree deletion example Delete 10 * 3 * 7 * 9 * a b d h e * 1 * * 4 * 5 * * 8 * * 12 * 13 * 12

13 File Types Regular files the ones that contain user information. Directories system files for maintaining the structure of the file system. Character special files used to model serial I/O devices direct access to hardware device. Block special files used to model disks (like hard disk). 13

14 Regular Files Text files (ASCII) line of text. The user/programs can easily see the data in the file and change them. Binary files Usually, they have some internal structure known to programs that use them. 14

15 Example of binary file Structure of executable file (from a version of Unix): Header: consist from identification number, size of any part of a file, the execution s start address and flags. Text and data of the program. Relocation bits. Symbol table for debugging. 15

16 An example program using file system calls copy file #include <sys/types.h> #include <fcntl.h> #include <stdlib.h> #include <unistd.h> #define BUF_SIZE 4096 #define OUTPUT_MODE

17 An example program using file system calls copy file int main(int argc, char *argv[]) { int in_fd, out_fd, rd_count, wt_count; char buffer[buf_size]; if (argc!= 3) exit(1); in_fd = open(argv[1], O_RDONLY); if (in_fd < 0) exit(2); out_fd = creat(argv[2], OUTPUT_MODE); if (out_fd < 0) exit(3); while(true) { rd_count = read(in_fd, buffer, BUF_SIZE); if(rd_count <= 0) break; wt_count = write(out_fd, buffer, rd_count); if (wt_count <= 0) exit(4); } close(in_fd); close(out_fd); if (rd_count == 0) exit(0); else exit(5); } 17

18 File System Directories 18

19 Directory system forms Single-Level Directory Systems Two-level Directory Systems Hierarchical Directory Systems 19

20 Specifying file names Absolute path name full path from root directory to the file. Relative path the path from working directory to the file. Working directory user s current directory. 20

21 File System Implementations 21

22 File System Layout File systems are stored on disk. A possible file system layout: 22

23 Implementing Files Contiguous Allocation: store each file as a contiguous run of disk blocks. What is the advantages and disadvantages of this implementation? 23

24 Implementing Files Linked List Allocation: Each file stored in a linked list of its fragments. 24

25 Implementing Files Linked List Allocation Using a Table in Memory 25

26 Implementing Files I-nodes (index node): Each file has a data structure called an i-node, which lists the attributes and disk addresses of the files blocks. 26

27 Implementing Directories Goal: mapping file s name onto the information needed to locate the data. Solution: keep a list of fixed-size entries, each entry contains file s name and its attributes or its i-node address. 27

28 Implementing Directories In some systems, The size of the file names is not fixed. How to improve the implementation? 28

29 29 Implementing Directories

30 Shared Files link connection between the user s folder and the shared file. What problems can be here? 30

31 Shared Files Link file A special file that represents a link to the shared file. symbolic linking Shared file s access via link files. 31