Filesystems

Question 1

Open-File Table

Answer 1

Purpose: avoid multiple searches of file directory structure

Contents: contains directory entries of the opened files

Question 2

Per-process open-file table

Answer 2

Contains entry per opened file

Every entry has a current-file pointer, as well as a pointer to the system-wide table

Question 3

System-wide open-file table

Answer 3

contains file info which is identical for all process

Question 4

3 implementations of Directory Structures

Answer 4

• Linear list
• Binary tree
• hash function

Question 5

Access Methods

Answer 5

• sequential access: read from first to last
• direct or relative access: give number

Question 6

Allocation method: Continuous Allocation

Answer 6

Each file has number of consecutive blocks.
+ fast sequential and direct access
- Problem: Dynamically increasing file size

Question 7

Allocation method: Linked/Chained Allocation

Answer 7

Each file forms a linked list of random blocks
+ fast sequential access
- slow direct access

Disadvantage:
- Possibly slow access
- lose number of bytes per block for pointer
=> resolve by forming clusters

Question 8

FAT: File Allocation Table

Answer 8

Variant of linked allocation
Fat contains entry per disk block

Question 9

Indexed Allocation

Answer 9

Each file has number of random blocks + index block
-> very fast direct access

Index is at least 1 block
-> large files = multiple blocks

Question 10

Free-space Management

Answer 10

• Bit vector: bit per block, 1 = free
  => too long for today’s systems
• Linked List => difficult to support continuous allocation
• Index solution: index to free blocks

Question 11

Journaling?

Answer 11

Quick recover the file system after a crash or power outage by specifying on disk what changes we want to make before we make them.

-> put transaction into log, perform, delete from log. After crash things in log? Perform!