FFS (Fast File System)
FFS, or the Fast File System, was introduced as an enhancement to the original Unix File System (UFS). Designed for performance and reliability, it provides better disk space utilization and faster access times compared to its predecessor. Here's a detailed look at FFS and its features:
Structure of the FFS File System
1. Superblock:
The superblock contains metadata about the file system, including:
File System Size:Total size of the file system in blocks.
Block Size:Size of each block (commonly 4096 or 8192 bytes).
Fragment Size:Minimum allocation size for small files.
Free Block Count:Tracks the number of free blocks.
Free Inode Count:Tracks the number of free inodes.
Cylinder Groups:Divides the file system into groups of cylinders for efficient allocation.
2. Cylinder Groups:
FFS divides the disk into cylinder groups, each containing:
Superblock Copies:
Backup of the file system's superblock for redundancy.
Block Bitmap:
Tracks the allocation status of blocks in the group.
Inode Bitmap:
Tracks the allocation status of inodes in the group.
Inode Table:
Stores metadata for files and directories.
Data Blocks:
Contains actual file and directory data.
3. Inode Structure:
Each file or directory in FFS is represented by an inode, which contains:
File Type and Permissions:
Indicates whether it's a file, directory, or special type.
File Size:
Size of the file in bytes.
Timestamps:
Tracks file creation, modification, and access times.
Pointers to Data Blocks:
Includes direct, indirect, double indirect, and triple indirect pointers.
4. Fragments:
FFS uses fragments to efficiently store small files. Fragments are smaller units within a block, reducing wasted space when storing small files.
Features of FFS
Cylinder Group Allocation:
Improves locality of reference by grouping related files within the same cylinder group.
Support for Large Volumes:
Handles larger volumes compared to the original Unix File System.
Efficient Space Utilization:
Uses fragments to minimize wasted space for small files.
Improved Performance:
Optimized disk layout reduces seek times.
Metadata Redundancy:
Stores multiple copies of the superblock for added reliability.
Limitations of FFS
Fragmentation:
Fragmentation can occur over time, requiring periodic maintenance.
Limited Scalability:
While robust for its time, FFS lacks modern features like journaling and snapshots.
Fixed Block and Fragment Sizes:
May lead to inefficiencies for certain workloads.
Tools and Commands for FFS
fsck:Checks and repairs FFS file systems.
newfs:Creates an FFS file system.
dump:Backs up data from an FFS file system.
tunefs:Adjusts tunable file system parameters.
Example
FFS was a significant advancement in file system design, providing improved performance and reliability over earlier systems. While it has been succeeded by more modern file systems, its principles continue to influence file system development.