1. File Systems and Disk Management
Andy Wang
Operating Systems
COP 4610 / CGS 5765

2. Design Goals of File Systems
Physical reality
File system abstraction
Block-oriented
Byte-oriented
Physical sectors
Named files
No protection
Users protected from one another
Data might be corrupted if machine crashes
Robust to machine failures

3. File System Components
Disk management organizes disk blocks into files
Naming provides file names and directories to users, instead of tracks and sector numbers (e.g. Takashita)
Protection keeps information secure from other users
Reliability protects information loss due to system crashes

4. User vs. System View of a File
User level: individual files
System call level: collection of bytes
Operating system level:
A block is a logical transfer unit
Even for getc() and putc()
4 Kbytes under UNIX
A sector is a physical transfer unit
512-byte sectors on disks
File: a named collection of blocks

5. User vs. System View of a File
A process
Read bytes 2 to 12 OS
Fetch the block containing those bytes
Return those bytes to the process

6. User vs. System View of a File
A process
Write bytes 2 to 12 OS
Fetch the block containing those bytes
Modify those bytes
Write out the block

7. Ways to Access a File People use file systems
Design of file systems involves understanding how people use file systems
Sequential access—bytes are accessed in order
Random access (direct access)—bytes are accessed in any order
Content-based access—bytes are accessed according to constraints on bye contents
e.g., return 100 bytes starting with “aye carumba”

8. File Usage Patterns
Most files are small, and most references are to small files
e.g., .login and .c files
Large files use up most of the disk space
e.g., mp3 files
Large files account for most of the bytes transferred between memory and disk
Bad news for file system designers

9. File System Design Constraints
High performance
Efficient access of small files
Many small files
Used frequently
Efficient access of large files
Consume most disk space
Account for most of the data movement

10. Some Definitions
A file contains a file header, which associates the file with its disk sectors
data block location
name
File header

11. Some Definitions
A file system needs a disk allocation bitmap to represent free space on the disk, one bit per block

12. Disk Allocation Policies
Contiguous allocation
Link-list allocation
Segment-based allocation
Indexed allocation
Multi-level indexed allocation
Hashed allocation

13. Contiguous Allocation
File blocks are stored contiguously on disk
To allocate a file,
Specify the file size
Search the disk allocation bitmap for consecutive free blocks
data block location
data block location
number of blocks
File header

14. Pros and Cons of Contiguous Allocation
+ Fast sequential access
+ Ease of computing random file locations
Adding an offset to the first disk block location
- External fragmentation
- Difficulty in growing files

15. Linked-List Allocation
Each file block on a disk is associated with a pointer to the next block
A special marker to indicate the end of the file
e.g., MS-DOS file system
File attribute table (FAT)
data block location
data block location
next block entry
next block entry
File header

16. Pros and Cons of Linked-List Allocation
+ Files can grow dynamically with incremental allocation of blocks
- Sequential access may suffer
Blocks may not be contiguous
- Horrible random accesses
May involve multiple sequential searches
- Unreliable
A corrupted pointer can lead to loss of the remaining file

17. Indexed Allocation
Uses a preallocated index to directly track the file block locations
data block location
data block location
data block location
File header

18. Pros and Cons of Indexed Allocation
+ Fast lookups and random accesses
- File blocks may be scattered all over the disk
Poor sequential access
Needs defragmenter
- Needs to reallocate index as the file size increases

19. Segment-Based Allocation
Needs a segment table to allocate multiple, contiguous regions of blocks
begin, end blocks
begin, end blocks
begin, end blocks
File header

20. Pros and Cons of Segment-Based Allocation
+ Relax the requirements for large contiguous disk regions
- Fragmentation  100%
Segment-based allocation  Indexed allocation
- Random accesses not as fast as pure contiguous allocation

21. Multilevel Indexed Allocation
Certain index entries point to index blocks, as opposed to data blocks (e.g., Linux ext2)
data block location 12
data block location
data block location
index block location
data block location
data block location
data block location
index block location
index block location
index block location
File header

22. Multilevel Indexed Allocation
A single indirect block contains pointers to data blocks
A double indirect block contains pointers to single indirect blocks
A triple indirect block contains pointers to double indirect blocks

23. Pros and Cons of Multilevel Indexed Allocation
+ Optimized for small and large files
Small files accessed through the first 12 pointers
Large files can grow incrementally
- Multiple disk accesses to fetch a data block under triple indirect block
- Largest file size capped by the number of pointers
- Arbitrary file size boundaries among levels

24. Hashed Allocation
Allocates a disk block by hashing the block content to a disk location
data block location
data block location
data block location
data block location
data block location
data block location
Old file header
New file header

25. Pros and Cons of Hashed Allocation
+ File blocks of the same content can share the same disk block to save storage
e.g., empty blocks
+ Good for backups and archival
Small modifications to a large file result in only additional storage of the changes
- Poor disk performance