ddrescue: Mastering Advanced Domain Control for Efficient Data Recovery

At revWhiteShadow, we understand the critical importance of data recovery, especially when dealing with failing hard drives and corrupted file systems. The journey to salvage every bit of your valuable data can be a complex one, often involving numerous attempts and meticulous configuration. We have personally navigated these intricate processes, learning firsthand how crucial it is to optimize ddrescue’s capabilities beyond basic usage. This article delves into advanced techniques to limit the rescue domain of ddrescue, focusing on used space and specific files, allowing for more efficient and targeted data recovery, particularly when faced with large drives or significant amounts of unusable space.

Our experience has shown that while initial commands to copy data from a problematic drive are essential, they can often be refined to save considerable time and wear on the failing hardware. We’ve encountered situations where, after initial passes, we realize that significant portions of the drive were empty or contained data we didn’t critically need. This realization often comes after hours, if not days, of a ddrescue process running. The key to overcoming this is leveraging ddrescue’s sophisticated domain mapping features to exclude free space and prioritize critical files.

Understanding ddrescue’s Core Functionality and the Need for Domain Control

ddrescue is a powerful command-line utility designed to recover data from failing drives. Unlike traditional dd, it intelligently handles read errors by skipping over bad sectors and retrying them later, minimizing stress on physically damaged media. This adaptive approach is its cornerstone. However, when dealing with drives that have extensive bad sectors or large amounts of unused space, a full disk scan can be incredibly time-consuming and may not be the most efficient path to recovery.

Our journey has led us to discover the potent, yet sometimes less documented, capabilities of ddrescue in defining specific areas to be rescued. This is where the concept of domain mapping becomes paramount. A domain in ddrescue refers to a range of blocks on the source device that the utility is instructed to process. By default, if no domain is specified, ddrescue attempts to process the entire device.

The Limitations of a Full-Scale Rescue

Imagine a terabyte drive with only a few gigabytes of actual data, but riddled with thousands of bad sectors scattered across the entire surface. A naive approach of rescuing the entire drive could result in hours of the system attempting to read unreadable sectors in the vast empty spaces, with little to no data being recovered from those areas. This not only wastes valuable time but also puts unnecessary strain on the already failing hardware, potentially exacerbating the damage and further reducing the chances of a successful recovery.

This is precisely the scenario where limiting the rescue domain becomes not just beneficial, but essential for an efficient and successful data recovery operation. By focusing ddrescue’s efforts on the areas known to contain data, or areas where specific critical files reside, we can significantly reduce the overall recovery time and improve the quality of the recovered data.

Leveraging ntfsbitmap for Initial Domain Mapping: A Crucial First Step

One of the most effective ways to exclude free disk space is by utilizing tools that can identify occupied blocks within a file system. For NTFS file systems, ntfsbitmap is an invaluable utility. It analyzes the NTFS bitmap, which essentially tracks which clusters on the disk are used by the file system and which are free. This information can then be translated into a ddrescue domain mapfile.

While we might have initially performed some rescue operations without this step, understanding its importance allows us to refine our approach. The ideal workflow would involve generating this mapfile before initiating the primary rescue. However, even if some data has already been copied, the ntfsbitmap mapfile can still be immensely useful for subsequent passes, especially when targeting specific files or the remaining unscraped areas.

Generating the ntfsbitmap Mapfile

The process of generating the mapfile typically involves:

  1. Identifying the Source Partition: This will be your problematic partition, e.g., /dev/sda1.
  2. Running ntfsbitmap: The command to generate the mapfile might look something like this (this is a conceptual example, as specific arguments can vary based on the ntfsbitmap version and system):
    ntfsbitmap -o ddru_ntfsbitmap.mapfile /dev/sda1
    This command instructs ntfsbitmap to analyze /dev/sda1 and output a mapfile named ddru_ntfsbitmap.mapfile. This mapfile will contain information about which blocks are allocated within the NTFS file system.

Applying the ntfsbitmap Mapfile with ddrescue

Once you have the ddru_ntfsbitmap.mapfile, you can instruct ddrescue to only process the blocks specified in this mapfile. This is done using the --domain-mapfile option.

Consider the following command, which builds upon our previous efforts:

sudo ddrescue \
  --idirect \
  --no-scrape \
  --sparse \
  --try-again \
  -r3 \
  --domain-mapfile=ddru_ntfsbitmap.mapfile \
  /dev/sda1 \
  output.img \
  output.logfile

Explanation of the parameters:

  • --idirect: Uses direct I/O, bypassing the system’s buffer cache. This can be beneficial for performance and consistency, especially on drives with many errors.
  • --no-scrape: This is crucial as we want to avoid the aggressive scraping phase, which can put significant strain on mechanical drives.
  • --sparse: Allows the output image to contain holes for unread blocks, saving disk space.
  • --try-again: As we previously considered, this flag is used to re-mark non-trimmed, non-scraped blocks as non-tried, ensuring they are revisited in subsequent passes.
  • -r3: Sets the number of retries for read errors to 3. You can adjust this value based on the drive’s condition and your patience.
  • --domain-mapfile=ddru_ntfsbitmap.mapfile: This is the key argument. It tells ddrescue to restrict its operations only to the blocks specified in the ddru_ntfsbitmap.mapfile. Any blocks not listed in this mapfile will be ignored, effectively skipping all free space.
  • /dev/sda1: The source device.
  • output.img: The output image file.
  • output.logfile: The logfile to record the rescue process.

By using the --domain-mapfile with the output from ntfsbitmap, we are effectively telling ddrescue to only focus on the areas of the disk that are known to be in use by the NTFS file system. This can dramatically shorten the rescue time and prevent unnecessary operations on empty sectors.

Targeting Specific Files with ntfsfindbad: Advanced Domain Refinement

The ntfsfindbad utility is another powerful tool in the ddrescue arsenal, particularly when you need to rescue specific files that are known to be affected by bad sectors. This utility scans the NTFS file system, identifies files that contain bad clusters, and reports their locations. The output, often found in a debug log, provides granular information about each affected file, including its inode, part, offset, full offset, size, type, and importantly, the errors and errorsize.

While ntfsfindbad provides the information, directly injecting this file-specific offset information into ddrescue’s main mapfile or using it as a --domain-mapfile requires a bit more manual effort or scripting. However, the principle remains the same: we want to create a domain that encompasses only the blocks relevant to our target files.

Interpreting ntfsfindbad Output for Domain Creation

The ntfsfindbad_debug.log file typically contains lines similar to this:

File: /path/to/some/important_document.docx, Inode: 12345, Part: 0, Offset: 1024, Fulloffset: 51200, Size: 4096, Type: Regular, Errors: 2, Errorsize: 512

Each line represents a file that has encountered issues. For our purpose, we are interested in the Fulloffset (the absolute byte offset from the beginning of the partition) and the Size of the file. To create a domain that covers these files, we need to translate this information into a format ddrescue understands for domain mapping.

ddrescue’s domain mapfile format typically consists of lines representing ranges of blocks:

start_block end_block

To use the information from ntfsfindbad, we would need to:

  1. Calculate the block ranges:

    • Determine the sector size of your drive (commonly 512 bytes).
    • Convert the Fulloffset and Size from bytes to sectors/blocks.
    • For each file, calculate the starting block and the ending block based on its Fulloffset and Size.
    • If a file spans multiple non-contiguous bad areas, ntfsfindbad might list them separately, or you might need to infer the total contiguous space occupied by the file for a simpler domain.

  2. Create a custom domain mapfile: Manually or through scripting, generate a new mapfile containing the calculated block ranges for the specific files you want to prioritize.

Let’s say we identified two important files and calculated their corresponding block ranges:

  • File 1 (Important Document): Occupies blocks 10000 to 10100.
  • File 2 (Critical Database): Occupies blocks 50000 to 55000.

Our custom domain mapfile (important_files.mapfile) would look like this:

10000 10100
50000 55000

Applying the Custom File-Specific Domain Mapfile

Now, we can use this custom mapfile with ddrescue:

sudo ddrescue \
  --idirect \
  --no-scrape \
  --sparse \
  --try-again \
  -r3 \
  --domain-mapfile=important_files.mapfile \
  /dev/sda1 \
  output.img \
  output.logfile

This command will instruct ddrescue to only attempt to read the blocks specified in important_files.mapfile. This is an incredibly precise way to recover specific data, especially when the rest of the drive’s contents are less critical or have already been processed.

Important Considerations:

  • Sector Size: Ensure you correctly account for the drive’s sector size when converting byte offsets to block numbers.
  • File System Alignment: Be mindful that file system structures can sometimes span across blocks in ways that require careful calculation.
  • Overlapping Ranges: If your targeted files have overlapping data ranges, ddrescue will handle them, but it’s good practice to consolidate ranges where possible for clarity.
  • Scripting: For numerous files, writing a script to parse the ntfsfindbad_debug.log and generate the mapfile is highly recommended.

Combining Domain Mapping Strategies: The Ultimate Efficiency

The true power of ddrescue’s domain control lies in combining these strategies. You might start with an ntfsbitmap mapfile to cover all used space, and then, if you have identified specific critical files with ntfsfindbad, you could create a more refined mapfile that only includes the blocks for those critical files.

Alternatively, you could run ddrescue with the ntfsbitmap mapfile first, to recover as much of the used space as possible. Then, if there are specific files you are particularly concerned about, you could generate a mapfile only for those files and run ddrescue again with that highly specific domain mapfile and potentially higher retry values (-r or -r0 for infinite retries, used with extreme caution).

Iterative Recovery with Different Domain Maps

Consider this iterative approach:

  1. Initial Broad Recovery of Used Space:

    sudo ddrescue --idirect --no-scrape --sparse --try-again -r3 --domain-mapfile=ddru_ntfsbitmap.mapfile /dev/sda1 output.img output.logfile

    This captures most of the data that is marked as used by the NTFS file system.

  2. Targeted Recovery of Critical Files (Post-Broad Recovery): Suppose after the first step, you still have some critical files that weren’t fully recovered, or you want to ensure their integrity. You’d generate a new mapfile, let’s call it super_critical_files.mapfile, specifically containing the block ranges for those files.

    sudo ddrescue \
  --idirect \
  --no-scrape \
  --sparse \
  --try-again \
  -r5 \
  --domain-mapfile=super_critical_files.mapfile \
  /dev/sda1 \
  output.img \
  output.logfile

    Here, we’ve increased retries (-r5) for potentially more stubborn areas within those critical files.

This layered approach allows you to balance the breadth of recovery with the precision needed for your most important data, all while minimizing unnecessary disk operations.

Addressing Your Specific Scenario: Integrating Past Efforts

Based on your description, you have already run ddrescue multiple times, achieving 99.99% rescued data with some unrescraped areas. Your plan to use the -A flag (--try-again) with sudo ddrescue --idirect -r3 --no-scrape --sparse --try-again /dev/sda1 output.img output.logfile is a sound one to attempt to recover those remaining non-scraped blocks.

Now, to your specific questions:

Can I use the ddru_ntfsbitmap mapfile at this stage?

Yes, absolutely! Even at this later stage, using the mapfile generated by ntfsbitmap is highly beneficial. If your output.img and output.logfile are already populated from previous runs, using the --domain-mapfile will tell ddrescue to only focus on the blocks listed in ddru_ntfsbitmap.mapfile that have not yet been successfully read (as recorded in output.logfile). This effectively skips reading empty disk areas for any remaining rescue operations.

So, your proposed command:

sudo ddrescue --idirect -r3 --no-scrape --sparse --try-again /dev/sda1 output.img output.logfile

can be modified to incorporate the domain mapfile:

sudo ddrescue \
  --idirect \
  --no-scrape \
  --sparse \
  --try-again \
  -r3 \
  --domain-mapfile=ddru_ntfsbitmap.mapfile \
  /dev/sda1 \
  output.img \
  output.logfile

This command will ensure that ddrescue only attempts to rescue blocks that are part of the NTFS file system’s used space and that haven’t been successfully read according to your output.logfile.

Can I further narrow the rescue domain using information from ntfsfindbad?

Yes, you can. If you are interested in only a few specific files that ntfsfindbad has identified as problematic, you can indeed create a highly targeted domain mapfile for them. This would involve parsing the ntfsfindbad_debug.log to extract the Fulloffset and Size for your files of interest, converting these to block ranges, and creating a new mapfile (e.g., critical_files.mapfile).

Then, you would use this new mapfile in your ddrescue command. This would be the most efficient way to rescue only those specific files that are most important to you, ignoring all other data, including free space and less important files.

The command would look like this:

sudo ddrescue \
  --idirect \
  --no-scrape \
  --sparse \
  --try-again \
  -r3 \
  --domain-mapfile=critical_files.mapfile \
  /dev/sda1 \
  output.img \
  output.logfile

This approach is particularly useful if the initial ntfsbitmap mapfile still results in a large number of blocks being processed, and you want to drastically reduce the scope to just a handful of critical files.

Conclusion: Mastering ddrescue for Optimized Data Recovery

Our journey with ddrescue has reinforced the understanding that its true power lies not just in its error-handling capabilities, but in its sophisticated domain mapping features. By strategically leveraging tools like ntfsbitmap and ntfsfindbad, we can create targeted rescue operations that significantly reduce recovery time, minimize wear on failing hardware, and maximize the chances of salvaging critical data.

The ability to limit the rescue domain to used space via ntfsbitmap and to specific files by manually crafting domain mapfiles based on ntfsfindbad output provides an unparalleled level of control over the data recovery process. Whether you are dealing with a partially failed drive or aiming for the most efficient salvage of specific data, these advanced techniques are invaluable.

At revWhiteShadow, we advocate for a proactive and informed approach to data recovery. Understanding and applying these ddrescue domain control methods will undoubtedly enhance your success rates and streamline the entire process. Remember, meticulous planning and precise command execution are key to navigating the complexities of failing media and achieving the best possible recovery outcomes.