Offline Direct Disk Access System

1
Off-line Direct Disk Access System

• CSE 237A
• Alan Del Tredici
• adeltred_at_ucsd.edu

2
Motivation

• Number of portable digital devices increasing
• Audio recorders
• Cameras
• Currently, data transferred to PC for
• Editing
• Storage / media transfer
• Network portal
• Requires
• 1. Boot PC
• 2. Transfer the data
• 3. Shut down PC

3
Proposed Solution

• Direct transfer to disk without booting PC

Offline Access Module
4
Project Goals

• Demonstrate off-line disk access procedure
• Write data to disk
• Without standard file I/O calls
• Without corrupting existing data
• Retrieve data via standard file I/O
• Proof-of-concept stage is software only

5
Development Choices

• Chose to use Linux as native OS
• Widely used, source code available, and free!
• Use Second Extended File System (Ext2)
• Unix-style file system
• De facto standard for Linux
• Basis from which other file systems extend

6
Development Choices (cont.)

• Make use of disk partitions
• Make 1 large disk look like several small
• Provides efficiency, modularity
• In Linux, floppy-disk accessed just like a
  hard-disk partition
• Use floppy disk as test target
• No change in concept
• Safer for development!

7
Off-line Access Strategy

• 1. Directly access disk to determine existing
  file system configuration
• 2. Add data, maintaining file system structure.
• 3. When booted, Linux will find the data

8
Development Approaches

• Write code to directly access disk
• Linux has strong protection user cant access
  hardware!
• Linux provides raw block read/write calls
• Want access to device in terms of file-system
  components
• This is provided by libext2fs.
• Basic functions to access file system structures
• Still requires us to know how to use the pieces

9
Ext2 Partition Components

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

Disk is an array of blocks organized by these
conventions
10
Ext2 Boot Sector

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

• Boot code
• May be unused
• Partition tables
• List start of partitions

11
Ext2 Super Block

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

• Basic parameters
• Block size (1 kB typ)
• Block counts
• Version info

12
Ext2 Group Descriptor

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

• Map table locations
• Handy counters

13
Ext2 Usage Maps

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

• Bit-maps where each bit indicates used/unused
  item
• A 1 kB block holds 8k bits
• Supports 8,192 entries

14
Ext2 Index nodes (i-nodes)

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

• Ext2 i-node 128 bytes
• struct ext2_inode
• U32 size
• U32 Block count
• U32 Time stamps
• U32 Block pointer0
• U32 Block pointer1
• U32 Block pointer11
• U32 Indirect block pointer
• U32 2x-indirect block pointer
• U32 3x-indirect block pointer

15
Ext2 I-node Block Pointers
16
Ext2 Data Blocks

• Boot sector
• 1 or more Groups
• Super block
• Group descriptor
• Block usage map
• I-node usage map
• I-node table
• Data blocks

• All remaining blocks in the group

17
Off-line Access Example

• Initialized Disk
• I-node table 23 blocks
• Root directory
• Directory is a type of file
• Index to other i-nodes by name
• Root i-node 2
• lostfound reserve 12 blocks

18
Off-line Access Steps

• Read configuration information
• Super block
• Block size (1 kbyte)
• I-node count (184 on fd)
• Block count (1440 on fd)
• Group block
• Location of maps
• Location of i-node table

19
File Creation

• Read the i-node and block maps
• Find the next available i-node and data block

20
File Creation (continued)

• Add entry to specified directory
• File name
• File type
• I-node number
• Perform i-node accounting
• Mark bit in i-node map

21
File Creation (continued)

• Write data to data blocks
• Update i-node with block numbers
• Allocate indirect blocks from data block area as
  needed

22
File Creation (continued)

• Finish configuring the i-node
• Set permissions
• Size of data written
• Time stamp
• When Linux reads this file system, our data will
  appear!

23
Accomplishments

• Met the technical objectives
• Data stored to hard disk without file I/O calls
• Linux can later retrieve data
• Documented pertinent Ext2 operation
• Extracted details from source code by testing
• Created working proof-of-concept application
• Available on web site
• http//ieng9.ucsd.edu/adeltred/offline_access.ht
  m

24
Future Work

• Test on hard disk with large files
• Implement on stand-alone hardware
• Atmel AT89C5132

A mass storage device controlling data exchange
between various Flash modules, HDD and CD-ROM.
25
Off-line Disk Access

• Questions?

26
Off-line Disk Access

• Thank you!