1. Contiguous Allocation of Disk Space

2. Linked Allocation

3. File-Allocation Table (DOS, others) Section of disk at start of partition Table with one entry per disk block Indexed by block# Each entry contains link to next block Special code for EOF “0” means empty block

4. Indexed Allocation  Brings all pointers (for one file) together into an index block.  Supports both sequential and random access (Index into table, pointer to block)  Wasteful for small files (use entire block for index) But no external fragmentation  Logical view. index table

5. Example of Indexed Allocation

6. Indexed Allocation – Multilevel Index (Supports very large file size)  outer-index index table file

7. Combined Scheme: UNIX inode (Example with 4K bytes per block) 12 link Size:blocks, bytes One inode table per partition 64 bytes per inode Directory entries point to inode Link count Direct index supports 48k file Double indirect supports huge files (>4 GB)

8. Maximum File Size with inodes  Assume 12 direct blocks, 4K blocks, 4 byte block pointer  Estimate maximum addressable up to the double indirect blocks: Direct blocks (12)48k Single indirect (4096/4=1024 ptrs to blocks)4096k Double indirect (1024 of these)4194304k Total4198448k (> 4 GB)  Didn’t even use triple indirect!

9. File Access With Unix inodes  “Superblock” and other structures not shown  “root” directory is inode #2  Directories contain pointers to inodes  Access path shown to /usr/bin/wc  Notice 8 disk accesses to get first block of wc! inodes (root) root directory usr directory bin directory wc (file) Data blocks usr bin wc #2 0 n

10. In-Memory File System Structures Re-Visited: (Unix Case) Opening A file Reading A file inode Copy of inode Pointer to Memory copy Of inode

11. Free-Space Management  Bit vector (n blocks) … 012n-1 bit[i] =  1  block[i] free 0  block[i] occupied To find a free block: find the first non-zero word, locate first 1-bit. Block number (of first free block) is: (number of bits per word) * (number of 0-value words) + offset of first 1 bit

12. Bit Vector (Cont.)  Bit map requires extra space. Example: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes)  Easy to get contiguous files Just look for consecutive 1’s in bit map

13. Linked List of Free Space on Disk

14. Free List Approach  Linked list Hard to find contiguous space easily But no waste of space  Grouping Store addresses of n free blocks in the first block Last of these addresses is to a block that contains addresses of another n free blocks So many free blocks can be found at one time  Counting Clusters of contiguous free blocks recorded together Keep list of first block address, count of contiguous free ones