1. Free Space Management

2. Bit Vector
Simple and efficient to find the first free block, or consecutive free blocks
By bit-manipulation
Requires extra space
block size = 212 bytes
disk size = 230 bytes
n = 230/212 = 218 bits (or 32K bytes)
Efficient only when the entire vector is kept in main memory
Write back to the disk occasionally for recovery needs 1 2
n-1 …
0  block[i] free
1  block[i] occupied
bit[i] =
 …
Question: What’s the block # of the fist free block?

3. Linked List Link together all free blocks
Keep a pointer to the first free block in a special location on the disk and caching it in memory
Cannot get contiguous space easily
No waste of space
Not efficient: have to traverse the disk for free spaces
Usually, OS needs one free block at a time
FAT incorporate the linked list mechanism

4. Grouping And Counting
Grouping: store the address of n free blocks in the first free block. The first n-1 are actually free. The final block contains the addresses of another n free blocks…
Counting: Each entry has a disk address and a count
Several contiguous blocks may be allocated or freed simultaneously

5. Example Of Free-Space Management
Bit Vector
Grouping Block 2  3, 4, 5 Block 5  8, 9, Block 10  11, 12, Block 13  17, 28, Block 25  26, 27
Counting

6. Efficiency and Performance

7. Efficiency and Performance
Efficiency dependent on
Disk allocation and directory algorithms
Types of data kept in file’s directory entry
Performance
On-board cache – local memory in disk controller to store entire tracks at a time
Disk cache – separate section of main memory for frequently used blocks (LRU is a reasonable algorithm for block replacement)
Free-behind and read-ahead – techniques to optimize sequential access (optimize the disk cache’s block replacement algorithm)
Improve PC performance by dedicating section of memory as virtual disk, or RAM disk.

8. Various Disk-Caching Locations

9. Page Cache Non-unified buffer cache Unified Buffer Cache
A page cache caches pages rather than disk blocks using virtual memory techniques
Memory-mapped I/O uses a page cache
Routine I/O through the file system uses the buffer (disk) cache
Unified Buffer Cache
A unified buffer cache uses the same buffer cache to cache both memory-mapped pages and ordinary file system I/O

10. I/O Without/With A Unified Buffer Cache

11. Recovery
Consistency checker – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies
Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape)
Recover lost file or disk by restoring data from backup

12. Log Structured File Systems
Log structured (or journaling) file systems record each update to the file system as a transaction
All transactions are written to a log. A transaction is considered committed once it is written to the log
However, the file system may not yet be updated
The transactions in the log are asynchronously written to the file system. When the file system is modified, the transaction is removed from the log
If the file system crashes, all remaining transactions in the log must still be performed