Ddrescue -
ddrescue is a raw disk imaging tool that "copies data from one file or block device to another, trying hard to rescue data in case of read errors." The application is developed as part of the GNU project and has written with UNIX/Linux in mind.
ddrescue and dd_rescue are completely different programs which share no development between them. The two projects are not related in any way except that they both attempt to enhance the standard dd tool and coincidentally chose similar names for their new programs.
From the ddrescue info pages:
GNU ddrescue is a data recovery tool. It copies data from one file or block device (hard disc, cdrom, etc) to another, trying hard to rescue data in case of read errors.
Ddrescue does not truncate the output file if not asked to. So, every time you run it on the same output file, it tries to fill in the gaps.
The basic operation of ddrescue is fully automatic. That is, you don't have to wait for an error, stop the program, read the log, run it in reverse mode, etc.
If you use the logfile feature of ddrescue, the data is rescued very efficiently (only the needed blocks are read). Also you can interrupt the rescue at any time and resume it later at the same point.
Automatic merging of backups: If you have two or more damaged copies of a file, cdrom, etc, and run ddrescue on all of them, one at a time, with the same output file, you will probably obtain a complete and error-free file. This is so because the probability of having damaged areas at the same places on different input files is very low. Using the logfile, only the needed blocks are read from the second and successive copies.
Installation
Bootable CD
ddrescue is available on bootable rescue cds such as SystemRescueCd https://www.system-rescue.org/.
Debian and Ubuntu
The package 'ddrescue' in Debian and Ubuntu is actually dd_rescue, another dd-like program which does not maintain a recovery log. The correct package is gddrescue.
Debian
aptitude install gddrescue
Ubuntu
sudo apt-get install gddrescue
Gentoo
emerge ddrescue
Partition recovery
Kernel 2.6.3+ & ddrescue 1.4+
'ddrescue --direct' will open the input with the O_DIRECT option for uncached reads. 'raw devices' are not needed on newer kernels. For older kernels see below.
First you copy as much data as possible, without retrying or splitting sectors:
ddrescue --no-split /dev/hda1 imagefile logfile
Now let it retry previous errors 3 times, using uncached reads:
ddrescue --direct --max-retries=3 /dev/hda1 imagefile logfile
If that fails you can try again but retrimmed, so it tries to reread full sectors:
ddrescue --direct --retrim --max-retries=3 /dev/hda1 imagefile logfile
You can now use ddrescue (or normal dd) to copy the imagefile to a new partition on a new disk. Use the appropriate filesystem checkers (fsck, CHKDSK) to try to fix errors caused by the bad blocks. Be sure to keep the imagefile around. Just in case the filesystem is severely broken, and datacarving tools like testdisk need to to be used on the original image.
Before linux kernel 2.6.3 / 2.4.x
In 2.6.3 the 'raw device' has been marked obsolete. On later kernels ddrescue will use O_DIRECT on the input to do uncached reads.
First you copy as much data as possible, without retrying or splitting sectors:
ddrescue --no-split /dev/hda1 imagefile logfile
Now change over to raw device access. Let it retry previous errors 3 times, don't read past last block in logfile:
modprobe raw raw /dev/raw/raw1 /dev/hda1 ddrescue --max-retries=3 --complete-only /dev/raw/raw1 imagefile logfile
If that fails you can try again (still using raw) but retrimmed, so it tries to reread full sectors:
ddrescue --retrim --max-retries=3 --complete-only /dev/raw/raw1 imagefile logfile
You can now use ddrescue (or normal dd) to copy the imagefile to a new partition on a new disk. Use the appropriate filesystem checkers (fsck, CHKDSK) to try to fix errors caused by the bad blocks. Be sure to keep the imagefile around. Just in case the filesystem is severely broken, and datacarving tools like testdisk need to to be used on the original image.
At the end you may want to unbind the raw device:
raw /dev/raw/raw1 0 0
Examples
These two examples are taken directly from the ddrescue info pages.
Example 1: Rescue an ext2 partition in /dev/hda2 to /dev/hdb2
Please Note: This will overwrite ALL data on the partition you are copying to. If you do not want to do that, rather create an image of the partition to be rescued.
ddrescue -r3 /dev/hda2 /dev/hdb2 logfile e2fsck -v -f /dev/hdb2 mount -t ext2 -o ro /dev/hdb2 /mnt
Example 2: Rescue a CD-ROM in /dev/cdrom
ddrescue -b 2048 /dev/cdrom cdimage logfile
write cdimage to a blank CD-ROM
This example is derived from the ddrescue manual.
Example 3: Rescue an entire hard disk /dev/sda to another disk /dev/sdb
copy the error free areas first
ddrescue -n /dev/sda /dev/sdb rescue.log
attempt to recover any bad sectors
ddrescue -r 1 /dev/sda /dev/sdb rescue.log
Options
-h, --help display this help and exit -V, --version output version information and exit -b, --block-size= hardware block size of input device [512] -B, --binary-prefixes show binary multipliers in numbers [default SI] -c, --cluster-size= hardware blocks to copy at a time [128] -C, --complete-only do not read new data beyond logfile limits -d, --direct use direct disc access for input file -D, --synchronous use synchronous writes for output file -e, --max-errors= maximum number of error areas allowed -F, --fill= fill given type areas with infile data (?*/-+) -g, --generate-logfile generate approximate logfile from partial copy -i, --input-position= starting position in input file [0] -n, --no-split do not try to split or retry error areas -o, --output-position= starting position in output file [ipos] -q, --quiet quiet operation -r, --max-retries= exit after given retries (-1=infinity) [0] -R, --retrim mark all error areas as non-trimmed -s, --max-size= maximum size of data to be copied -S, --sparse use sparse writes for output file -t, --truncate truncate output file -v, --verbose verbose operation
Numbers may be followed by a multiplier: b = blocks, k = kB = 10^3 = 1000, Ki = KiB = 2^10 = 1024, M = 10^6, Mi = 2^20, G = 10^9, Gi = 2^30, etc...
Cygwin
As of release 1.4-rc1, it can be compiled directly in Cygwin Out of the Box. Precompiled packages are available in the Cygwin distribution. This makes it usable natively on Windows systems.
See also
aimage
Blackbag
dcfldd
dd
dd_rescue