Lecture 19: File Systems 1

Question 1

Character devices (keyboard, display, modem)

Answer 1

• input or output 1 character at a time

- data transfer: byte-at-a-time

Question 2

Block devices (disk drive, tape drive)

Answer 2

• input/output data in fixed size blocks

- data transfer: block-at-a-time

Question 3

Block

Answer 3

-amount of data read in one READ operation

Question 4

Files

Answer 4

logically contiguous, non-volatile, protected collection of related data

Question 5

Directories

Answer 5

special-purpose files that contain information about other files

Question 6

File Systems

Answer 6

• allows for efficient and convenient access to disk
• resides on disk
• organized into layers

Question 7

I/O Control

Answer 7

– lowest layer of OS abstraction
– consists of device drivers (software layer between
application and device controller)
– translates commands such as: retrieve block 123
to hardware-specific instructions used by controller

Question 8

Basic file system

Answer 8

– issues generic commands to device driver to read, write blocks
– block-level addressing

Question 9

File organization module

Answer 9

– translates logical block addresses (file id + block offset) to physical block addresses
(drive #, cylinder #, track #, sector #)

– includes free space manager for allocating, deallocating disk blocks

Question 10

Logical file system

Answer 10

– manages metadata about files (e.g., size, permissions)

– maintains with file control blocks

Question 11

File system data structures (on disk)

Answer 11

1. Boot control block
   • 1 per bootable partition
   • info needed to boot OS from partition (bootstrap brings into RAM)
   • stored in fixed location within partition (e.g., 1st block)
2. partition control block
   • 1 per partition
   • includes info about partition:
   – # blocks, size of blocks, free-block count, free-block pointers, …
3. directory structure blocks
   • 1 per directory
   • file organization information for directory

4. file control block 
• 1 per file
• typical contents:
– permissions
– timestamps (created, last access, last write)
– owner
– group
– size
– data block locations...

Question 12

File system data structures (memory)

Answer 12

1. PartitionTable
   • information about all mounted (visible) partitions
2. DirectoryStructureTable
   •information about recently accessed directories
3. Open File Tables
   a) SystemOpenFileTable:
   • copies of file control blocks (FCB’s) for every open file system-wide b) Process Open File Tables:
   • lists of pointers to system open file entries for files opened by a process

Question 13

Block Allocation

Answer 13

Comparing the Schemes

1. Contiguous
   - every file occupies a set of “contiguous” blocks on disk

+: good sequential, random access
-: poor disk utilization, files bounded in size

2. Linked
   file = list of disk blocks which can be scattered on disk

+: good sequential access, disk utilization, unbounded file size -: poor random access

3. Indexed
   • use dedicated “index blocks” to store pointers to data blocks

+: good disk utilization, unbounded file size,
good sequential, random access of few levels in index -: overhead of index blocks
access performance worsens with more levels

Question 14

Free space management: Bit vector

Answer 14

• one bit per block
+: easy to find free blocks
easy to find contiguous blocks
-: can get very large, yet should be kept in memory

Question 15

Free space management: linked list

Answer 15

• maintain pointer to 1st free block, which contains pointer to next free block, which contains pointer to next free block…

+: simple implementation little space overhead
-: traversing list is expensive, but typically you only need:

• 1st block in list: to allocate a frame
• last block in list: to deallocate a frame (keep 2nd pointer)