dd_rescue Command: Tutorial & Examples

Copy data from one device to another

dd_rescue is a powerful command-line utility in Unix-like operating systems such as Linux. It is used to copy data from one file or block device (hard disk, CD-ROM, etc.) to another. It's particularly useful when encountering damaged media where standard copy operations fail.

dd_rescue is often used to recover data that has been accidentally deleted or from damaged storage devices. It can bypass bad sectors on a disk, creating a copy of the data that can be recovered, hence its significance.

How dd_rescue Works

The dd_rescue command works by copying data from the source to the target at a raw level. Unlike the copy command cp for files or the standard dd command for block devices, dd_rescue does not stop when it encounters errors. Instead, it skips over problematic areas and continues copying. This ability to work around errors makes it an exceptional tool for data recovery situations.

The command uses two passes to complete data recovery. In the first pass, it skips over problematic areas quickly to prevent further damage. In the second pass, it goes back to recover as much data as possible from the problematic areas.

Typical Problems Solved by dd_rescue

One of the most common problems dd_rescue can solve is data recovery from a failing hard drive. When a hard drive starts to fail, it may develop bad sectors that prevent normal reading and writing. dd_rescue can bypass these bad sectors and retrieve as much data as possible.

Another problem that can be addressed by dd_rescue is recovery from optical media like CDs and DVDs. These media types can become scratched or otherwise damaged, making normal data retrieval difficult. dd_rescue, with its error bypassing ability, can often recover data from these sources when other methods fail.

Examples of dd_rescue Usage

Using dd_rescue involves specifying a source and a destination. Here's a simple example:

dd_rescue /dev/sda1 /home/user/sda1_image

In this example, /dev/sda1 is the source (a partition on the first hard disk), and /home/user/sda1_image is the destination (an image file in the user's home directory).

If you're dealing with a damaged disk, you can use the -r option to make dd_rescue retry failed sectors:

dd_rescue -r 3 /dev/sda1 /home/user/sda1_image

This command tells dd_rescue to retry failed sectors three times before giving up.

Typical Output of dd_rescue

When you run a dd_rescue command, it provides output that updates you on its progress. You'll see the amount of data that's been copied, the current data rate, and the number of errors encountered.

Here's what a typical output might look like:

dd_rescue: (info): ipos: 1052672.0k, opos: 1052672.0k, xferd: 1052672.0k
             errs:   0, errxfer:    0.0k, succxfer: 1052672.0k
       +curr.rate:   12354kB/s, avg.rate:   12354kB/s, avg.load: 99.9%

In this example, dd_rescue has transferred around 1GB of data at an average rate of approximately 12MB/s, and it has not encountered any errors.

Conclusion

Whether you're dealing with a failing hard drive or damaged optical media, dd_rescue is a powerful tool that can help you recover as much data as possible. Its ability to bypass errors and continue copying sets it apart from standard copy commands. With practice and understanding, dd_rescue can become a crucial part of your Linux toolkit.