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
dd_rescue command works by copying data from the source to the target at a raw level. Unlike the copy
cp for files or the standard
dd command for block
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
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.
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.