1. SE350: Operating Systems
Lecture 12: File Systems

2. Main Points
File layout
Directory layout

3. Design Challenges Index structure Index granularity Free space maps
How do we locate the blocks of a file?
Index granularity
What block size do we use?
Free space maps
How do we find unused blocks on disk?
Locality heuristics
How do we group data to optimize performance?

4. Named Data in a File System

5. Directories Are Files
Directory is a file that contains a collection of file name → file number mappings
Can we use open/close/read/write API to access directories?
No; kernel should prevent apps from corrupting the file name → file number mappings

6. Recursive Filename Lookup
Directories can be arranged hierarchically
A directory can contain the file name → file number mapping for another directory

7. Directory Layout Original Linux ext2 File System
Linked list implementation of a directory
Linear search is required to find filename
Reasonable implementation for small directories
What if there are a few thousand entries?

8. Directory Layout Modern Linux XFS File System

9. Directory Layout Modern Linux XFS File System (cont.)

10. Hard and Soft Links

11. Microsoft File Allocation Table (FAT)
FAT is an array of 32-bit entries
Each file corresponds to linked list of FAT entries
Simple and easy to implement
Still widely used
Each FAT entry corresponds to a storage block

12. FAT Pros: Cons: Easy to find free block Easy to append to a file
Easy to delete a file
Cons:
Random access is very slow
Fragmentation and poor locality
File blocks for a given file may be scattered
Files in the same directory may be scattered
Problem becomes worse as disk fills

13. Berkeley UNIX FFS (Fast File System)
Metadata
File owner, access permissions, access times, …
Set of 12 data pointers
With 4KB blocks => max size of 48KB
Indirect block pointer
Pointer to disk block of data pointers
4KB block size and 4-byte block pointer => 1K direct block pointers => 4MB data (+48kB)
Doubly indirect block pointer
Doubly indirect block => 1K indirect blocks
4GB (+ 4MB + 48KB)
Triply indirect block pointer
Triply indirect block => 1K doubly indirect blocks
4TB (+ 4GB + 4MB + 48KB)

14. FFS Inode and Inode Array

15. FFS Asymmetric Tree Small files: shallow tree Large files: deep tree
Efficient storage for small files
Large files: deep tree
Efficient lookup for random access in large files
Sparse files: only fill pointers if needed

16. FFS Locality Block group placement inode table spread throughout disk
Block group is a set of nearby tracks
Files in same directory located in same group
Subdirectories located in different block groups
inode table spread throughout disk
inodes, bitmap near file blocks
First fit allocation
Small files fragmented, large files contiguous

17. FFS Block Groups

18. FFS First Fit Block Allocation

19. FFS First Fit Block Allocation

20. FFS First Fit Block Allocation

21. FFS Pros Cons Efficient storage for both small and large files
Locality for both small and large files
Locality for metadata and data
Cons
Inefficient for tiny files (a 1 byte file requires both an inode and a data block)
Inefficient encoding when file is mostly contiguous on disk (no equivalent to superpages)
Need to reserve 10-20% of free space to prevent fragmentation

22. Acknowledgment
This lecture is a slightly modified version of the one prepared by Tom Anderson