Module 13: The OS File System

Question 1

System calls for files:

Answer 1

fopen(): open file for reading and writing
fwrite(): writes data to a file
fseek(): sets the position of the stream to the given offset, allowing seeking a specified part of the file
fread(): read from the file
fclose(): close the file

Question 2

disk block

Answer 2

basic logic unit of storage on disk of a contiguous set of bytes

Question 3

disk partitions

Answer 3

logical split of the actual disk drive; the OS views each partition as a distinct disk, and different OS may be run in each split; each partition has at least one directory, in which all the partition’s files are listed

Question 4

typical internal structure of the disk parition:

Answer 4

1. boot block: contains info on the disk layout and code for loading the OS into the kernel space
2. superblock: contains basic information about the file system, including the file system size, list of free blocks, list of allocated blocks, and time of the last modification to the partition; the superblock can be read into memory at boot time
3. free space data structure: tracks the free space in the disk
4. i-nodes list: stores information about the individual files stored on disk

Question 5

master boot record

Answer 5

stores information about the entire drive

Question 6

partition table

Answer 6

specifies the beginning of each partition

Question 7

disk space allocation

Answer 7

managing data in the disk with the aims to ensure fast sequential success, fast random access, the ability to grow a file system quickly, and to minimize fragmentation

Question 8

4 methods for managing disk space allocation

Answer 8

1. contiguous allocation
2. linked-list allocation
3. file allocation table (FAT)
4. linked list with indexing or i-nodes
   - -> i-node: a kernel structure storing information about each file in the file system; contains a pointer to the disk blocks containing the file’s data, and information such as the file permissions, ownership, modification time, and file type

Question 9

directory

Answer 9

a file that holds the list of filenames and their i-nodes; provides information needed to find the disk data blocks of a file

Question 10

hard links

Answer 10

to represent two different paths for a file, create a single i-node and have two directory entries that point to the same i-node; relies on the link-counter

link-counter: counts the number of ways that an i-node can be retrieved along different paths

Question 11

symbolic/soft link

Answer 11

original file has its own i-node and for each reference or path to the file, there is a separate i-node of type LINK, for a file that just contains the path name to the original file

Question 12

free space management

Answer 12

tracking unallocated blocks in a portion of the disk partition

• -> bitmap: maintains the entire disk partition with one encoding the fact that the block of a particular number is free and zero otherwise
• -> linked list: storing the free blocks

Question 13

in-memory cache

Answer 13

uses a scheme to store blocks that are likely to be requested from disk, to shortcut the need for a disk search and propagate the data to the requesting process
–> LRU scheme: given the limited in-memory cache size, evicts the least recently used block from the cache when new blocks are being added in

Question 14

incremental backups

Answer 14

every time you synchronize with your server, it will only backup files that have been changed since the last time

Question 15

physical backup

Answer 15

physically copy block by block from one disk to another

Question 16

logical backup

Answer 16

copies files from one disk to another