Disk image tools are essential utilities for creating, managing, and manipulating disk images. These images are bit-for-bit copies of a storage device (like a hard drive, USB stick, or CD/DVD) and are used for various purposes, including backups, OS deployment, virtualization, and data recovery. This article provides an in-depth look at the most widely used disk image tools, their features, and how they can be effectively utilized.

What Are Disk Images?

A disk image is a file that contains the complete contents and structure of a storage device. Disk images can be of different formats such as ISO, IMG, VHD, and others. They are used to replicate the data exactly as it appears on the original storage device, including the file system, boot records, and all files.

Why Use Disk Image Tools?

• Backup and Recovery: Disk images are used for creating backups and for restoring systems to a previous state.
• OS Deployment: Deploying operating systems on multiple machines can be streamlined using disk images.
• Virtualization: Virtual machines often use disk images to store their virtual hard disks.
• Data Recovery: Disk images can be used to recover data from corrupted or failing storage devices.

Popular Disk Image Tools

1 dd

The dd command is a versatile and powerful utility in Unix-like operating systems. It is primarily used for low-level copying and conversion of raw data from one location to another. This command can be employed for tasks such as creating disk images, writing boot loaders, performing data recovery, and much more.

Functions and Uses of dd

1. Disk Cloning and Imaging: Creating exact bit-for-bit copies of disks or partitions.
2. Backup and Restore: Making backups of entire disks or partitions and restoring them.
3. Boot Loader Installation: Writing boot loader code to the master boot record (MBR).
4. Data Conversion: Converting data formats and performing transformations.
5. Performance Testing: Measuring read/write performance by creating large files.
6. Data Recovery: Extracting data from damaged disks or partitions.

Syntax of dd

The basic syntax of the dd command is:

dd if=<input_file> of=<output_file> [options]

• if=<input_file>: Specifies the input file or device.
• of=<output_file>: Specifies the output file or device.
• [options]: Additional options to control the operation.

Common Options

• bs=<block_size>: Sets the block size for both input and output operations. Example: bs=512
• count=<number_of_blocks>: Limits the operation to a specified number of input blocks. Means specifies the number of blocks to copy.
• skip=<number_of_blocks>: Skips a specified number of input blocks while reading the input file
• seek=<number_of_blocks>: Skips a specified number of output blocks while writing to the output file.
• conv=<conversion_options>: Specifies conversions to be applied to the data. Examples: conv=notrunc (do not truncate output file), conv=sync (pad blocks to the full block size).

Detailed Examples in OS Development :-

1 Creating a Disk Image:

To create an image of an entire disk, you can use dd as follows:

dd if=/dev/sda of=/path/to/backup.img bs=4M

2 Creating an Empty Disk Image:

To create an empty disk image, you specify the size using the count and bs options. This example creates a 10 MB disk image:

dd if=/dev/zero of=disk.img bs=1M count=10

• if=/dev/zero: Use /dev/zero as the input file, which provides null bytes.
• of=disk.img: Output file is disk.img.
• bs=1M: Block size is 1 megabyte.
• count=10: Create 10 blocks of 1 MB each, totaling 10 MB.

3 Writing Binary Files to a Disk Image:

You can write binary files, such as bootloaders and kernels, to specific locations in the disk image. For example, to write a stage1 bootloader, a stage2 bootloader, and a kernel:

– Write stage1.bin at the beginning (MBR):

dd if=stage1.bin of=disk.img bs=512 seek=0 conv=notrunc

• if=stage1.bin: Input file is stage1.bin.
• of=disk.img: Output file is disk.img.
• bs=512: Block size of 512 bytes (standard sector size).
• seek=0: Write at the beginning (sector 0).
• conv=notrunc: Do not truncate the output file.

– Write stage2.bin starting at sector 1:

dd if=stage2.bin of=disk.img bs=512 seek=1 conv=notrunc

• seek=1: Start writing at sector 1.

– Write kernel.bin starting at sector 59:

dd if=kernel.bin of=disk.img bs=512 seek=59 conv=notrunc

• seek=59: Start writing at sector 59.