File Systems and Disk Management

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

2. Design Goals of File Systems

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 • 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. data block location data block location name Some Definitions • A file contains a file header, which associates the file with its disk sectors 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. data block location data block location number of blocks 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 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. data block location next block entry data block location next block entry 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) 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. data block location data block location data block location data block location Indexed Allocation • Uses a preallocated index to directly track the file block locations 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. begin, end blocks begin, end blocks begin, end blocks begin, end blocks Segment-Based Allocation • Needs a segment table to allocate multiple, contiguous regions of 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. index block location index block location data block location index block location index block location index block location index block location data block location data block location data block location data block location data block location data block location data block location 12 Multilevel Indexed Allocation • Certain index entries point to index blocks, as opposed to data blocks (e.g., Linux ext2) 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. data block location data block location data block location data block location data block location data block location data block location data block location Hashed Allocation • Allocates a disk block by hashing the block content to a disk 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 performance