Linux - ddrescue

The following is a Linux Dictionary word of the day: ddrescue.

GNU ddrescue is a powerful Linux data‑recovery utility that copies data from one block device to another while intelligently handling read errors. Unlike the classic dd command, ddrescue:

• Skips bad sectors automatically
• Retries only the damaged areas
• Uses a log file to track progress
• Can resume recovery at any time

This makes it ideal for failing HDDs, SSDs, USB drives, SD cards, and corrupted partitions.

1. -d parameter

• Direct disk access mode
• Forces ddrescue to bypass the OS cache
• Reads directly from the hardware

Useful for failing drives where caching may cause delays or hangs

Use when: recovering from physically damaged or unstable disks.

2. -f parameter

• Force writing to a block device
• Required when the output target is a raw device (e.g., /dev/sdb)
• Prevents accidental overwrites by requiring explicit confirmation
• Use when: cloning disk → disk.

3. -r[n] parameter

Retry bad sectors

• -r tells ddrescue to retry failed reads
• -r3 retries 3 times
• -r0 means no retries
• -r-1 means retry forever
• Used when: you want to squeeze every last readable byte from a failing disk.

Here’s a clean, production‑ready ddrescue command:

ddrescue -d -f /dev/mdev /dev/mdev2 ddclone.log

Breakdown:

-d → direct disk access

-f → force writing to block device

/dev/mdev → source disk

/dev/mdev2 → destination disk

ddclone.log → log file tracking recovery progress

Why the log file matters

If the recovery is interrupted, you can resume instantly:

ddrescue -d -f /dev/mdev /dev/mdev2 ddclone.log

ddrescue reads the log and continues exactly where it left off.

Best Practices for Safe Data Recovery

• Always clone first, never work on the failing disk directly
• Use a separate physical disk as the destination
• Keep the log file on a healthy drive
• Avoid mounting the failing disk
• Use -r3 or -r5 for moderate retries
• For severely damaged drives, use -r-1 (infinite retries) cautiously

Compared to other tools, ddrescue stands out because it:

• Automatically handles bad sectors
• Optimizes read order for maximum recovery
• Uses a persistent log file
• Works on any block device
• Is fast, reliable, and open‑source
• For Linux admins, technicians, and digital forensics specialists, ddrescue is simply indispensable.

Key Takeaways

ddrescue is the safest and most efficient Linux tool for disk cloning and recovery

• Use -d for direct disk access
• Use -f when writing to block devices
• Use -r[n] to retry bad sectors
• Always create and preserve a log file