W4118 Operating Systems
Instructor: Junfeng Yang
File System Examples
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Unix Fast File System (FFS)
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Linux Second Extended File System (EXT2)
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What were the problems with Unix FS?
How did FFS solve these problems?
What is the EXT2 on-disk layout?
What is the EXT2 directory structure?
Virtual File System (VFS)
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What is VFS?
What are the key data structures of Linux VFS?
1
Original Unix FS
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From Bell Labs
Simple and elegant
Problem: slow
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20KB/s, 2% of maximum even for sequential disk
transfer
inodes
super block
data blocks (512 bytes)
Why so slow?
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Blocks too small
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Unorganized freelist
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Fixed costs per transfer (seek and rotational
delays)
Need more indirect blocks
Consecutive file blocks are not close together
Pay seek cost even for sequential access
No data locality
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inodes far from data blocks
inodes of files in directory not close together
Larger Blocks
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BSD: increase block to 4096 or more bytes
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Faster
However, internal fragmentation
FFS provided a solution
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Not a problem now: disks are big and cheap
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Unorganized freelist
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Leads to random allocation of sequential file
blocks overtime
Initial performance good
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Get worse over time
Solution: bitmap for free blocks
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E.g., 1000011100
Data Locality
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Store related data together
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Always find free block nearby
• Keep free space on disks (10%)
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Spread unrelated data apart
• Make room for related data
How to Achieve Locality
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Each cylinder group contains
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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
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Spread out directories to cylinder groups
Switch to a different cylinder group for large files
File System Examples

Unix Fast File System (FFS)



Linux Second Extended File System (EXT2)



What were the problems with Unix FS?
How did FFS solve these problems?
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
Ext2
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“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
Ext2 Disk Layout
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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. 0-13-6006639
Block Addressing in Ext2
Twelve “direct” blocks
Inode
Data
Data
Block
Data
Block
Block
BLKSIZE/4
Indirect
Blocks
Indirect
Blocks
(BLKSIZE/4)2
Double
Indirect
Indirect
Blocks
(BLKSIZE/4)3
Triple
Indirect
Double
Indirect
Indirect
Blocks
Indirect
Blocks
Data
Data
Block
Data
Block
Block
Data
Data
Block
Data
Block
Block
Data
Data
Block
Data
Block
Block
Data
Data
Block
Data
Block
Block
Data
Data
Block
Data
Block
Block
Picture from Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
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. 0-13-6006639
File System Examples

Unix Fast File System (FFS)



Linux Second Extended File System (EXT2)



What were the problems with Unix FS?
How did FFS solve these problems?
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
VFS
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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
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A VFS specifies an interface
A specific FS implements this interface
• Often a struct of function pointers
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VFS dispatches FS operations through this
interface
• E.g., dir->inode_op->mkdir();
Schematic View of Virtual File
System
Key Linux VFS Data Structures
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struct file
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struct dentry
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information about an open file
includes current position (file pointer)
information about a directory entry
includes name + inode#
struct inode
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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
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struct superblock
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descriptor of a mounted filesystem