GNU ddrescue Manual

This manual is for GNU ddrescue (version 1.16, 11 June 2012).

• Introduction: Purpose and features of GNU ddrescue
• Basic Concepts: Blocks, clusters, devices, files, sectors, etc
• Algorithm: How ddrescue recovers the data
• Invoking Ddrescue: Command line interface
• Logfile Structure: Detailed format of the logfile
• Examples: A small tutorial with examples
• Direct Disc Access: Bypassing the kernel cache
• Fill Mode: Selectively overwriting the output file
• Generate Mode: Generating an approximate logfile
• Ddrescuelog: Tool for ddrescue logfiles
• Invoking Ddrescuelog: Command line interface
• Problems: Reporting bugs
• Concept Index: Index of concepts

This manual is free documentation: you have unlimited permission to copy, distribute and modify it.

1 Introduction

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.

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 backwards, 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.

Ddrescue does not write zeros to the output when it finds bad sectors in the input, and 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 without wiping out the data already rescued.

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.

Ddrescue recommends lzip for compression of backups because of its reliability and data recovery capabilities, including error-checked merging of backup copies. The combination ddrescue + lziprecover is the best option for recovering data from multiple damaged copies. See lziprecover-example, for an example.

Recordable CD and DVD media keep their data only for a finite time (typically for many years). After that time, data loss develops slowly with read errors growing from the outer media region towards the inside. Just make two (or more) copies of every important CD/DVD you burn so that you can later recover them with ddrescue.

Because ddrescue needs to read and write at random places, it only works on seekable (random access) input and output files.

If your system supports it, ddrescue can use direct disc access to read the input file, bypassing the kernel cache.

Ddrescue also features a "fill mode" able to selectively overwrite parts of the output file, which has a number of interesting uses like wiping data, marking bad areas or even, in some cases, "repair" damaged sectors.

2 Basic Concepts

Block

Any amount of data. A block is described by its starting position and its size.

Cluster

Group of consecutive sectors read or written in one go.

Device

Piece of hardware containing data. Hard disc drives, cdrom drives, USB pendrives, are devices. /dev/hda, /dev/sdb, are device names.

File

Files are named units of data which are stored by the operating system for you to retrieve later by name. Devices are accessed by means of their associated file names.

Partition

Every part in which a device is divided. A partition normally contains a file system. /dev/hda1, /dev/sdb3, are partition names.

Recoverable formats

As ddrescue uses standard library functions to read data from the device being rescued, only mountable device formats can be rescued with ddrescue. DVDs can be mounted and they can be rescued, "compact disc digital audio" CDs can't, "video CDs"[1] maybe.
[1] http://en.wikipedia.org/wiki/Video_CD

Rescue domain

Block or set of blocks to be acted upon (rescued, listed, etc). You can define it with the options `--input-position', `--max-size' and `--domain-logfile'. The rescue domain defaults to the whole input file or logfile.

Sector

Hardware block. Smallest accessible amount of data on a device.

3 Algorithm

GNU ddrescue manages efficiently the status of the rescue in progress and tries to rescue the good parts first, scheduling reads inside bad (or slow) areas for later. This maximizes the amount of data that can be finally recovered from a failing drive.

The standard dd utility can be used to save data from a failing drive, but it reads the data sequentially, which may wear out the drive without rescuing anything if the errors are at the beginning of the drive.

Other programs switch to small size reads when they find errors, but they still read the data sequentially. This is a bad idea because it means spending more time at error areas, damaging the surface, the heads and the drive mechanics, instead of getting out of them as fast as possible. This behavior reduces the chances of rescuing the remaining good data.

The algorithm of ddrescue is as follows (the user may interrupt the process at any point, but be aware that a bad drive can block ddrescue for a long time until the kernel gives up):

1) Optionally read a logfile describing the status of a multi-part or previously interrupted rescue. If no logfile is specified or is empty or does not exist, mark all the rescue domain as non-tried.

2) Read the non-tried parts of the input file, marking the failed blocks as non-trimmed and skipping beyond them, until all the rescue domain is tried. Only non-tried areas are read in large blocks. Trimming, splitting and retrying are done sector by sector. Each sector is tried at most two times; the first in this step as part of a large block read, the second in one of the steps below as a single sector read.

3) Read backwards one sector at a time the non-trimmed blocks, until a bad sector is found. For each non-trimmed block, mark the bad sector found as bad-sector and mark the rest of that block as non-split.

4) Read forwards one sector at a time the non-split blocks, marking the bad sectors found as bad-sector. After a number of consecutive bad sectors is found in a block large enough, the block is split by half and the reading continues on the second half. This recursively splits the largest failed blocks without producing a logfile too large.

5) Optionally try to read again the bad sectors until the specified number of retries is reached.

6) Optionally write a logfile for later use.

The logfile is periodically saved to disc, as well as when ddrescue finishes or is interrupted. So in case of a crash you can resume the rescue with little recopying.

Also, the same logfile can be used for multiple commands that copy different areas of the input file, and for multiple recovery attempts over different subsets. See this example:

Rescue the most important part of the disc first.

     ddrescue -i0 -s50MiB /dev/hdc hdimage logfile
     ddrescue -i0 -s1MiB -d -r3 /dev/hdc hdimage logfile

Then rescue some key disc areas.

     ddrescue -i30GiB -s10GiB /dev/hdc hdimage logfile
     ddrescue -i230GiB -s5GiB /dev/hdc hdimage logfile

Now rescue the rest (does not recopy what is already done).

     ddrescue /dev/hdc hdimage logfile
     ddrescue -d -r3 /dev/hdc hdimage logfile

4 Invoking Ddrescue

The format for running ddrescue is:

     ddrescue [options] infile outfile [logfile]

ddrescue supports the following options:

`-h'

`--help'

Print an informative help message describing the options and exit.

`-V'

`--version'

Print the version number of ddrescue on the standard output and exit.

`-a bytes'

`--min-read-rate=bytes'

Minimum read rate of good non-tried areas, in bytes per second. If the read rate falls below this value, ddrescue will skip ahead a variable amount depending on rate and error history. The skipped blocks are tried in additional passes (before trimming) where the minimum read rate is divided by ten before each pass, until there are no more non-tried blocks left. If bytes is 0 (auto), the minimum read rate is recalculated for each block read as (average_rate / 10).

Note that specifying a minimum read rate above the posibilities of the input device will result in a very low average rate because all reads will be considered slow reads and continuous skipping will occur. This will also make the logfile grow disproportionately.

`-A'

`--try-again'

Mark all non-split and non-trimmed blocks inside the rescue domain as non-tried before beginning the rescue. Try this if the drive stops responding and ddrescue immediately starts splitting failed blocks when restarted. If `--retrim' is also specified, mark all failed blocks inside the rescue domain as non-tried.

`-b bytes'

`--block-size=bytes'

Sector (hardware block) size of input device in bytes (usually 512 for hard discs and 3.5" floppies, 1024 for 5.25" floppies, and 2048 for cdroms). Defaults to 512.

`-B'

`--binary-prefixes'

Show units with binary prefixes (powers of 1024).
SI prefixes (powers of 1000) are used by default. (See table below).

`-c sectors'

`--cluster-size=sectors'

Number of sectors to copy at a time. Defaults to 64 KiB / sector_size. Try smaller values for slow drives. The number of sectors per track (18 or 9) is a good value for floppies.

`-C'

`--complete-only'

Limit rescue domain to the blocks listed in the logfile. Do not read new data beyond logfile limits. This is useful when reading from devices of undefined size, like raw devices, or when the drive returns an incorrect size. It can only be used after a first rescue attempt, possibly limited with the `--max-size' option, has produced a complete logfile.

`-d'

`--direct'

Use direct disc access to read from infile, bypassing the kernel cache. (Open the file with the O_DIRECT flag). Use it only on devices or partitions, not on regular files. Sector size must be correctly set for this to work. Not all systems support this.

If your system does not support direct disc access, ddrescue will warn you. If the sector size is not correctly set, all reads will result in errors, and no data will be rescued.

`-D'

`--synchronous'

Use synchronous writes for outfile. (Issue a fsync call after every write). May be useful when forcing the drive to remap its bad sectors.

`-e [+]n'

`--max-errors=[+]n'

Maximum number of error areas allowed before giving up. Defaults to infinity. If n is preceded by `+' the number refers to new error areas found in this run, not counting those already annotated in the logfile.

`-E bytes'

`--max-error-rate=bytes'

Maximum rate of errors allowed before giving up, in bytes per second. Defaults to infinity. The rate being measured is that of actually failed reads, so the rescue may finish because of this rate being exceeded even if the total error size (errsize) does not change because the areas being tried are already marked as errors.

`-f'

`--force'

Force overwrite of outfile. Needed when outfile is not a regular file, but a device or partition.

`-F types'

`--fill=types'

Fill the blocks in outfile specified as any of types in logfile, with data read from infile. types contains one or more of the status characters defined in the chapter Logfile Structure (see Logfile Structure). See the chapter Fill Mode (see Fill Mode) for a complete description of the fill mode.

`-g'

`--generate-logfile'

Generate an approximate logfile from the infile and outfile of the original rescue run. Note that you must keep the original offset between `--input-position' and `--output-position' of the original rescue run. See the chapter Generate Mode (see Generate Mode) for a complete description of the generate mode.

`-i bytes'

`--input-position=bytes'

Starting position in infile, in bytes. Defaults to 0. In fill mode it refers to a position in the infile of the original rescue run. See the chapter Fill Mode (see Fill Mode) for details.

`-I'

`--verify-input-size'

Compare the size of infile with the size calculated from the list of blocks contained in the logfile, and exit with status 1 if they differ. This is not enabled by default because the size of some devices can't be known in advance and because the size derived from the logfile may be incomplete, for example after doing a partial rescue.

`-K bytes'

`--skip-size=bytes'

Set the initial size to skip on the first read error or slow read. The value given will be rounded to the next multiple of sector size. For subsequent read errors or slow reads, ddrescue will modify the skip size depending on error rate and error history. Valid values range from 64 KiB to 1 GiB. Defaults to 64 KiB.

`-m file'

`--domain-logfile=file'

Restrict the rescue domain to the blocks marked as finished in the logfile file. This is useful if the destination drive fails during the rescue.

`-M'

`--retrim'

Mark all failed blocks inside the rescue domain as non-trimmed before beginning the rescue. The effect is similar to `--max-retries=1', but the bad sectors are tried in a different order, making perhaps possible to rescue some of them.

`-n'

`--no-split'

Skip the splitting pass. Avoids spending a lot of time trying to rescue the most difficult parts of the file.

`-o bytes'

`--output-position=bytes'

Starting position in outfile, in bytes. Defaults to `--input-position'. The bytes below bytes aren't touched if they exist and truncation is not requested. Else they are set to 0.

`-p'

`--preallocate'

Preallocate space on disc for outfile. Only space for regular files can be preallocated. If preallocation succeeds, rescue will not fail due to lack of free space on disc. If ddrescue can't determine the size to preallocate, you may need to specify it with some combination of the `--input-position', `--output-position', `--max-size', and `--domain-logfile' options.

`-q'

`--quiet'

Quiet operation. Suppress all messages.

`-r n'

`--max-retries=n'

Exit after given number of retry passes. Defaults to 0. -1 means infinity. Every bad sector is tried only one time per pass. To retry bad sectors detected on a previous run, you must specify a non-zero number of retries.

`-R'

`--reverse'

Reverse direction of all copy operations. This makes copying, splitting and retrying to be run backwards, and trimming to be run forwards.

`-s bytes'

`--max-size=bytes'

Maximum size of the input data to be copied, in bytes. If ddrescue can't determine the size of the input device, you may need to specify it with this option. Note that this option specifies the size of the input data to be copied, not the size of the resulting outfile. So, for example, the following command creates an outfile 300 bytes long, but only writes data on the last 200 bytes:

          ddrescue -i 100 -s 200 infile outfile logfile
     

`-S'

`--sparse'

Use sparse writes for outfile. (The blocks of zeros are not actually allocated on disc). May save a lot of disc space in some cases. Not all systems support this. Only regular files can be sparse.

`-t'

`--truncate'

Truncate outfile to zero size before writing to it. Only works for regular files, not for drives or partitions.

`-T interval'

`--timeout=interval'

Maximum time since last successful read allowed before giving up. Defaults to infinity. interval is a rational number (like 1.5 or 1/2) optionally followed by one of `s', `m', `h' or `d', meaning seconds, minutes, hours and days respectively. If no unit is specified, it defaults to seconds.

`-v'

`--verbose'

Verbose mode. Further -v's (up to 4) increase the verbosity level.

`-x bytes'

`--extend-outfile=bytes'

Extend the size of outfile to make it at least bytes long. If the size of outfile is already equal or longer than bytes then this option does nothing. Use this option to guarantee a minimum size for outfile. Only regular files can be extended.

Table of SI and binary prefixes (unit multipliers):

Prefix	Value	|	Prefix	Value
		|	b	hardware blocks
k	kilobyte (10^3 = 1000)	|	Ki	kibibyte (2^10 = 1024)
M	megabyte (10^6)	|	Mi	mebibyte (2^20)
G	gigabyte (10^9)	|	Gi	gibibyte (2^30)
T	terabyte (10^12)	|	Ti	tebibyte (2^40)
P	petabyte (10^15)	|	Pi	pebibyte (2^50)
E	exabyte (10^18)	|	Ei	exbibyte (2^60)
Z	zettabyte (10^21)	|	Zi	zebibyte (2^70)
Y	yottabyte (10^24)	|	Yi	yobibyte (2^80)

5 Logfile Structure

The logfile is a text file easy to read and edit. It is formed by three parts, the heading comments, the status line, and the list of data blocks.

Any line beginning with `#' is a comment line. The blocks in the list of data blocks must be contiguous and non-overlapping.

NOTE: Logfiles generated by a version of ddrescue prior to 1.6 lack the status line. If you want to use an old logfile with ddrescue 1.6 or later, you will have to insert a line like `0 +' at the beginning of the logfile.

The heading comments contain the version of ddrescue and the command line used to create the logfile. They are intended as information for the user.

The first non-comment line is the status line. It contains a non-negative integer and a status character. The integer is the position being tried in the input file. The status character is one of these:

Character	Meaning
'?'	copying non-tried blocks
'*'	trimming non-trimmed blocks
'/'	splitting non-split blocks
'-'	retrying bad sectors
'F'	filling specified blocks
'G'	generating approximate logfile
'+'	finished

Every line in the list of data blocks describes a block of data. It contains 2 non-negative integers and a status character. The first integer is the starting position of the block in the input file, the second integer is the size (in bytes) of the block. The status character is one of these:

Character	Meaning
'?'	non-tried block
'*'	failed block non-trimmed
'/'	failed block non-split
'-'	failed block bad-sector(s)
'+'	finished block

And here is an example logfile:

# Rescue Logfile. Created by GNU ddrescue version 1.16
# Command line: ddrescue /dev/fd0 fdimage logfile
# current_pos current_status
0x00120000 ?
# pos size status

0x00000000	0x00117000	+
0x00117000	0x00000200	-
0x00117200	0x00001000	/
0x00118200	0x00007E00	*
0x00120000	0x00048000	?

If you edit the file, you may use decimal, hexadecimal or octal values, using the same syntax that integer constants in C++.

6 A small tutorial with examples

Ddrescue is like any other power tool. You need to understand what it does, and you need to understand some things about the machines it does those things to, in order to use it safely.

A failing drive tends to develop more and more errors as time passes. Because of this, you should rescue the data from a drive as soon as you notice the first error. Be diligent because every time a physically damaged drive powers up and is able to output some data, it may be the very last time that it ever will.

You should make a copy of the failing drive with ddrescue, and then try to repair the copy. If your data is really important, use the first copy as a master for a second copy, and try to repair the second copy. If something goes wrong, you have the master intact to try again.

IMPORTANT! Always use a logfile unless you know you won't need it. Without a logfile, ddrescue can't resume a rescue, only reinitiate it.

IMPORTANT! Never try to rescue a r/w mounted partition. The resulting copy may be useless.

IMPORTANT! If you use a device or a partition as destination, any data stored there will be overwritten.

IMPORTANT! If you interrupt the rescue and then reboot, any partially copied partitions should be hidden before allowing them to be touched by any operating system that tries to mount and "fix" the partitions it sees.

IMPORTANT! Never try to repair a file system on a drive with I/O errors; you will probably lose even more data.

If you are trying to rescue a whole partition, first repair the copy with e2fsck or some other tool appropiate for the type of partition you are trying to rescue, then mount the repaired copy somewhere and try to recover the files in it.

If the drive is so damaged that the file system in the rescued partition can't be repaired or mounted, you will have to browse the rescued data with an hex editor and extract the desired parts by hand or use a file recovery tool like photorec.

If the partition table is damaged, you may try to rescue the whole disc, then try to repair the partition table and the partitions on the copy.

If the damaged drive is not listed in /dev, then you cannot rescue it. At least not with ddrescue.

     ddrescue -f -n /dev/hda /dev/hdb logfile
     ddrescue -d -f -r3 /dev/hda /dev/hdb logfile
     fdisk /dev/hdb
     e2fsck -v -f /dev/hdb1
     e2fsck -v -f /dev/hdb2

     ddrescue -f -n /dev/hda2 /dev/hdb2 logfile
     ddrescue -d -f -r3 /dev/hda2 /dev/hdb2 logfile
     e2fsck -v -f /dev/hdb2
     mount -t ext2 -o ro /dev/hdb2 /mnt
       (read rescued files from /mnt)

     ddrescue -n -b2048 /dev/cdrom cdimage logfile
     ddrescue -d -b2048 /dev/cdrom cdimage logfile
       (write cdimage to a blank CD-ROM)

     ddrescue -n -b2048 /dev/cdrom cdimage logfile
     ddrescue -d -b2048 /dev/cdrom cdimage logfile
       (insert second copy in the CD drive)
     ddrescue -d -r1 -b2048 /dev/cdrom cdimage logfile
       (write cdimage to a blank CD-ROM)

     ddrescue -b2048 /dev/cdrom cdimage1 logfile1
     mount -t iso9660 -o loop,ro cdimage1 /mnt/cdimage
     cp /mnt/cdimage/backup.tar.lz rescued1.tar.lz
     umount /mnt/cdimage
       (insert second copy in the CD drive)
     ddrescue -b2048 /dev/cdrom cdimage2 logfile2
     mount -t iso9660 -o loop,ro cdimage2 /mnt/cdimage
     cp /mnt/cdimage/backup.tar.lz rescued2.tar.lz
     umount /mnt/cdimage
     lziprecover -m -v -o rescued.tar.lz rescued1.tar.lz rescued2.tar.lz

     ddrescue -f -n /dev/hda /dev/hdb logfile1     <-- /dev/hdb fails here
     ddrescue -f -m logfile1 /dev/hdb /dev/hdc logfile2
     ddrescue -f -n /dev/hda /dev/hdc logfile2
     ddrescue -d -f -r3 /dev/hda /dev/hdc logfile2

     ddrescue -f -n /dev/hda /dev/hdb logfile      <-- /dev/hda fails here
       (restart /dev/hda or reboot computer as many times as needed)
     ddrescue -f -n -A /dev/hda /dev/hdb logfile
     ddrescue -d -f -r3 /dev/hda /dev/hdb logfile

7 Direct Disc Access

If you notice that the positions and sizes in the log file are ALWAYS multiples of the sector size, maybe your kernel is caching the disc accesses and grouping them. In this case you may want to use direct disc access or a raw device to bypass the kernel cache and rescue more of your data.

NOTE! Sector size must be correctly set with the `--block-size' option for this to work. Try the `--direct' option first. If direct disc access is not available in your system, try raw devices. Read your system documentation to find how to bind a raw device to a regular block device.

Ddrescue aligns its I/O buffer to the sector size so that it can be used for direct disc access or to read from raw devices. For efficiency reasons, also aligns it to the memory page size if page size is a multiple of sector size. Ddrescue can't determine the size of a raw device, so an explicit `--max-size' or `--complete-only' option is needed.

Using direct disc access, or reading from a raw device, may be slower or faster than normal cached reading depending on your OS and hardware. In case it is slower you may want to make a first pass using normal cached reads and use direct disc access, or a raw device, only to recover the good sectors inside the failed blocks.

     ddrescue -f -n /dev/hdb1 /dev/hdc1 logfile
     ddrescue -d -f -r3 /dev/hdb1 /dev/hdc1 logfile
     e2fsck -v -f /dev/hdc1
     mount -t ext2 -o ro /dev/hdc1 /mnt

     raw /dev/raw/raw1 /dev/hdb1
     ddrescue -f -n /dev/hdb1 /dev/hdc1 logfile
     ddrescue -C -f -r3 /dev/raw/raw1 /dev/hdc1 logfile
     raw /dev/raw/raw1 0 0
     e2fsck -v -f /dev/hdc1
     mount -t ext2 -o ro /dev/hdc1 /mnt

8 Fill Mode

When ddrescue is invoked with the `--fill' option it operates in "fill mode", which is different from the default "rescue mode". That is, if you use the `--fill' option, ddrescue does not rescue anything. It only fills with data read from the input file the blocks of the output file whose status character from the logfile coincides with one of the type characters specified as argument to the `--fill' option.

In fill mode the input file may have any size. If it is too small, the data will be duplicated as many times as necessary to fill the input buffer. If it is too big, only the data needed to fill the input buffer will be read. Then the same data will be written to every block or sector to be filled.

Note that in fill mode the input file is always read from position 0. If you specify a `--input-position', it refers to the original input file from which the logfile was built, and is only used to calculate the offset between input and output positions.

Note also that when filling the input file of the original rescue run you should set `--input-position' and `--output-position' to identical values, whereas when filling the output file of the original rescue run you should keep the original offset between `--input-position' and `--output-position'.

The `--fill' option implies the `--complete-only' option.

In fill mode the logfile is updated to allow resumability when interrupted or in case of a crash, but as nothing is being rescued the logfile is not destroyed. The status line is the only part of the logfile that is modified.

Example 1: Mark parts of the rescued copy to allow finding them when examined in an hex editor. For example, the following command line fills all blocks marked as `-' (bad-sector) with copies of the string `BAD SECTOR ':

     printf "BAD SECTOR " > tmpfile
     ddrescue --fill=- tmpfile outfile logfile

Example 2: Wipe only the good sectors, leaving the bad sectors alone. This way, the drive will still test bad (i.e., with unreadable sectors). This is the fastest way of wiping a failing drive, and is specially useful when sending the drive back to the manufacturer for warranty replacement.

     ddrescue --fill=+ --force /dev/zero bad_drive logfile

Example 3: Force the drive to remap the bad sectors, making it usable again. If the drive has only a few bad sectors, and they are not caused by drive age, you can probably just rewrite those sectors, and the drive will reallocate them automatically to new "spare" sectors that it keeps for just this purpose. WARNING! This may not work on your drive.

     ddrescue --fill=- --force --synchronous /dev/zero bad_drive logfile

1) Copy the damaged drive with ddrescue until finished. Do not use sparse writes. This yields a logfile with only finished (`+') and bad-sector (`-') blocks.

2) Fill the bad-sector blocks of the copied drive or image file with a string not present in any file, for example "DEADBEEF".

3) Mount the copied drive (or the image file, via loopback device).

4) Grep for the fill string in all the files. Those files containing the string reside (at least partially) in damaged disc areas.

5) Unmount the copied drive or image file.

6) Optionally fill the bad-sector blocks of the copied drive or image file with zeros to restore the disc image.

Example 4: Figure out what files are in the bad areas of the disc.

     ddrescue -b2048 /dev/cdrom cdimage logfile
     printf "DEADBEEF" > tmpfile
     ddrescue --fill=- tmpfile cdimage logfile
     rm tmpfile
     mount -t iso9660 -o loop,ro cdimage /mnt/cdimage
     find /mnt/cdimage -type f -exec grep "DEADBEEF" '{}' ';'
     umount /mnt/cdimage
     ddrescue --fill=- /dev/zero cdimage logfile

9 Generate-logfile Mode

So you didn't read the tutorial and started ddrescue without a logfile. Now, two days later, your computer crashed and you can't know how much data ddrescue managed to save. And even worse, you can't resume the rescue; you have to restart it from the very beginning.

Or maybe you started copying a drive with `dd conv=noerror,sync' and are now in the same situation described above. In this case, note that you can't use a copy made by dd unless it was invoked with the `sync' conversion argument.

Don't despair (yet). Ddrescue can in some cases generate an approximate logfile, from the input file and the (partial) copy, that is almost as good as an exact logfile. It makes this by simply assuming that sectors containing all zeros were not rescued.

However, if the destination of the copy was a drive or a partition, (or an existing regular file and truncation was not requested), most probably you will need to restart ddrescue from the very beginning. (This time with a logfile, of course). The reason is that old data may be present in the drive that have not been overwritten yet, and may be thus non-tried but non-zero.

For example, if you first tried one of these commands:

     ddrescue infile outfile
     or
     dd if=infile of=outfile conv=noerror,sync

then you can generate an approximate logfile with this command:

     ddrescue --generate-logfile infile outfile logfile

Note that you must keep the original offset between `--input-position' and `--output-position' of the original rescue run.

10 Ddrescuelog

Ddrescuelog is a tool that manipulates ddrescue logfiles, shows logfile contents, converts logfiles to/from other formats, compares logfiles, tests rescue status, and can delete a logfile if the rescue is done. Ddrescuelog operations can be restricted to one or several parts of the logfile if the domain setting options are used.

Here are some examples of how to use ddrescuelog, alone or in combination with other tools.

     ddrescue -f /dev/hda /dev/hdb logfile
     ddrescuelog -d logfile

     fdisk /dev/hdb                                <-- partition /deb/hdb
     ddrescue -f /dev/hda1 /dev/hdb1 logfile1
     ddrescue -f /dev/hda2 /dev/hdb2 logfile2
     ddrescuelog -l- -b4096 logfile1 > badblocks1
     ddrescuelog -l- -b4096 logfile2 > badblocks2
     e2fsck -v -f -L badblocks1 /dev/hdb1
     e2fsck -v -f -L badblocks2 /dev/hdb2

     ddrescue -f /dev/hda /dev/hdb logfile
     fdisk /dev/hdb                                <-- get partition sizes
     ddrescuelog -l- -b512 -i63b -o0 -s9767457b -b4096 logfile > badblocks1
     ddrescuelog -l- -b512 -i9767520b -o0 -s128520b -b4096 logfile > badblocks2
     e2fsck -v -f -L badblocks1 /dev/hdb1
     e2fsck -v -f -L badblocks2 /dev/hdb2

11 Invoking Ddrescuelog

The format for running ddrescuelog is:

     ddrescuelog [options] logfile

Ddrescuelog supports the following options:

`-h'

`--help'

Print an informative help message describing the options and exit.

`-V'

`--version'

Print the version number of ddrescuelog on the standard output and exit.

`-a old_types,new_types'

`--change-types=old_types,new_types'

Change the status of every block in the rescue domain from one type in old_types to the corresponding type in new_types, much like the `tr' command does, and write the resulting logfile to standard output. old_types and new_types are strings of block status characters as defined in the chapter Logfile Structure (see Logfile Structure). Blocks whose status is not in old_types are left unchanged. If new_types is shorter than old_types the last type of new_types is repeated as many times as necessary.

`-b bytes'

`--block-size=bytes'

Block size used by ddrescuelog. Depending on the requested operation it may be the sector size of the input device, the block size of the rescued file system, etc. Defaults to 512.

`-c[type1type2]'

`--create-logfile[=type1type2]'

Create a logfile from a list of block numbers read from standard input. Only blocks included in the rescue domain will be added to logfile.

type1 and type2 are block status characters as defined in the chapter Logfile Structure (see Logfile Structure). type1 sets the type for blocks included in the list, while type2 sets the type for the rest of the logfile. If not specified, type1 defaults to `+' and type2 defaults to `-'.

`-d'

`--delete-if-done'

Delete the given logfile if all the blocks in the rescue domain have been successfuly recovered. The exit status is 0 if logfile could be deleted, 1 otherwise.

`-D'

`--done-status'

Test if all the blocks in the rescue domain have been successfuly recovered. The exit status is 0 if all tested blocks are finished, 1 otherwise.

`-f'

`--force'

Force overwrite of logfile.

`-i bytes'

`--input-position=bytes'

Starting position of the rescue domain, in bytes. Defaults to 0. It refers to a position in the original input file.

`-l types'

`--list-blocks=types'

Print on standard output the block numbers of the blocks specified as any of types in logfile and included in the rescue domain. types contains one or more of the block status characters defined in the chapter Logfile Structure (see Logfile Structure).

The list format is one block number per line in decimal, like the output of the badblocks program, so that it can be used as input for e2fsck or other similar filesystem repairing tool.

`-m file'

`--domain-logfile=file'

Restrict the rescue domain to the blocks marked as finished in the logfile file.

`-n'

`--invert-logfile'

Invert the types of the blocks in logfile which are included in the rescue domain, and write the resulting logfile to standard output. Finished blocks (`+') are changed to bad-sector (`-'), all other types are changed to finished. `--invert-logfile' is equivalent to `--change-types=?*/-+,++++-'

`-o bytes'

`--output-position=bytes'

Starting position in output file, in bytes. Is used by the `--list-blocks' option. Defaults to `--input-position'.

`-p file'

`--compare-logfile=file'

Compare the types of the blocks included in the rescue domain. The exit status is 0 if all tested blocks are the same in both files, 1 otherwise.

`-q'

`--quiet'

Quiet operation. Suppress all messages.

`-s bytes'

`--max-size=bytes'

Maximum size of the rescue domain, in bytes. It refers to a size in the original input file.

`-t'

`--show-status'

Print a summary of logfile contents on the standard output. The summary can be restricted to one or several parts of logfile if the domain setting options are used.

`-v'

`--verbose'

Verbose mode. Further -v's (up to 4) increase the verbosity level.

`-x file'

`--xor-logfile=file'

Perform a logical XOR (exclusive OR) operation between the finished blocks in file and those in logfile, and write the resulting logfile to standard output. In other words, in the resulting logfile a block is only shown as finisned if it was finisned in either of the two input logfiles but not in both.

`-y file'

`--and-logfile=file'

Perform a logical AND operation between the finished blocks in file and those in logfile, and write the resulting logfile to standard output. In other words, in the resulting logfile a block is only shown as finisned if it was finisned in both input logfiles.

`-z file'

`--or-logfile=file'

Perform a logical OR operation between the finished blocks in file and those in logfile, and write the resulting logfile to standard output. In other words, in the resulting logfile a block is shown as finisned if it was finisned in either of the two input logfiles.

12 Reporting Bugs

There are probably bugs in ddrescue. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.

If you find a bug in GNU ddrescue, please send electronic mail to bug-ddrescue@gnu.org. Include the version number, which you can find by running `ddrescue --version'.

Concept Index

• algorithm: Algorithm
• basic concepts: Basic Concepts
• bugs: Problems
• ddrescuelog: Ddrescuelog
• direct disc access: Direct Disc Access
• examples: Examples
• Fill Mode: Fill Mode
• Generate-logfile Mode: Generate Mode
• getting help: Problems
• introduction: Introduction
• invoking ddrescue: Invoking Ddrescue
• invoking ddrescuelog: Invoking Ddrescuelog
• logfile structure: Logfile Structure
• options: Invoking Ddrescue
• raw devices: Direct Disc Access
• usage: Invoking Ddrescue
• version: Invoking Ddrescue