0. Instructor: Junfeng Yang
W4118 Operating Systems
Instructor: Junfeng Yang

1. File System Examples Unix Fast File System (FFS)
What were the problems with Unix FS?
How did FFS solve these problems?
Linux Second Extended File System (EXT2)
What is the EXT2 on-disk layout?
What is the EXT2 directory structure?
Virtual File System (VFS)
What is VFS?
What are the key data structures of Linux VFS?

2. Original Unix FS From Bell Labs Simple and elegant Problem: slow
20KB/s, 2% of maximum even for sequential disk transfer
inodes
data blocks (512 bytes)
super block

3. Why so slow? Blocks too small Unorganized freelist No data locality
Fixed costs per transfer (seek and rotational delays)
Need more indirect blocks
Unorganized freelist
Consecutive file blocks are not close together
Pay seek cost even for sequential access
No data locality
inodes far from data blocks
inodes of files in directory not close together

4. Larger Blocks BSD: increase block to 4096 or more bytes Faster
However, internal fragmentation
FFS provided a solution
Not a problem now: disks are big and cheap

5. Initial performance good
Unorganized freelist
Leads to random allocation of sequential file blocks overtime
Solution: bitmap for free blocks
E.g.,
Initial performance good
Get worse over time

6. Data Locality Store related data together
Always find free block nearby
Keep free space on disks (10%)
Spread unrelated data apart
Make room for related data

7. How to Achieve Locality
Each cylinder group contains
Superblock
Inodes
Bitmap of free blocks
Additional info for block allocation
Data blocks
File: store data blocks within a cylinder group
Dir: store files in same dir in a cylinder group
Make room
Spread out directories to cylinder groups
Switch to a different cylinder group for large files

8. File System Examples Unix Fast File System (FFS)
What were the problems with Unix FS?
How did FFS solve these problems?
Linux Second Extended File System (EXT2)
What is the EXT2 on-disk layout?
What is the EXT2 directory structure?
Virtual File System (VFS)
What is VFS?
What are the key data structures of Linux VFS? 8

9. Ext2 “Standard” Linux File System
Previously it was the most commonly used
Serves as a basis for Ext3 which adds journaling
Uses FFS like layout
Each file system is composed of identical block groups
Allocation is designed to improve locality
inodes contain pointers (32 bits) to blocks
Direct, Indirect, Double Indirect, Triple Indirect
Maximum file size: 4.1TB (4K Blocks)
Maximum file system size: 16TB (4K Blocks)
On-disk structures defined in include/linux/ext2_fs.h

10. Ext2 Disk Layout
Files in the same directory are stored in the same block group
Files in different directories are spread among the block groups
Picture from Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

11. Block Addressing in Ext2
Twelve “direct” blocks
Data
Block
Inode
Data
Block
Data
Block
BLKSIZE/4
Indirect
Blocks
Data
Block
Data
Block
Data
Block
Data
Block
Indirect
Blocks
(BLKSIZE/4)2
Data
Block
Data
Block
Double
Indirect
Indirect
Blocks
Data
Block
Data
Block
Data
Block
Data
Block
(BLKSIZE/4)3
Data
Block
Indirect
Blocks
Data
Block
Triple
Indirect
Double
Indirect
Data
Block
Indirect
Blocks
Data
Block
Data
Block
Picture from Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

12. Ext2 Directory Structure
(a) A Linux directory with three files. (b) The same directory after the file voluminous has been removed.
Picture from Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved

13. File System Examples Unix Fast File System (FFS)
What were the problems with Unix FS?
How did FFS solve these problems?
Linux Second Extended File System (EXT2)
What is the EXT2 on-disk layout?
What is the EXT2 directory structure?
Virtual File System (VFS)
What is VFS?
What are the key data structures of Linux VFS? 13

14. VFS Old days: “the” file system
Nowadays: many file system types and instances co-exist
VFS: an FS abstraction layer that transparently and uniformly supports multiple file systems
A VFS specifies an interface
A specific FS implements this interface
Often a struct of function pointers
VFS dispatches FS operations through this interface
E.g., dir->inode_op->mkdir();

15. Schematic View of Virtual File System

16. Key Linux VFS Data Structures
struct file
information about an open file
includes current position (file pointer)
struct dentry
information about a directory entry
includes name + inode#
struct inode
unique descriptor of a file or directory
contains permissions, timestamps, block map (data)
inode#: integer (unique per mounted filesystem)
Pointer to FS-specific inode structure
e.g. ext2_inode_info
struct superblock
descriptor of a mounted filesystem