\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename ddrescue.info
@settitle GNU ddrescue Manual
@finalout
@c %**end of header

@set UPDATED 10 January 2011
@set VERSION 1.14

@dircategory GNU Packages
@direntry
* ddrescue: (ddrescue).         Data recovery tool
@end direntry


@ifnothtml
@titlepage
@title GNU ddrescue
@subtitle Data recovery tool
@subtitle for ddrescue version @value{VERSION}, @value{UPDATED}
@author by Antonio Diaz Diaz

@page
@vskip 0pt plus 1filll
@end titlepage

@contents
@end ifnothtml

@node Top
@top

This manual is for GNU ddrescue (version @value{VERSION}, @value{UPDATED}).

@menu
* 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
* Problems::            Reporting bugs
* Concept Index::       Index of concepts
@end menu

@sp 1
Copyright @copyright{} 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
Antonio Diaz Diaz.

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


@node Introduction
@chapter Introduction
@cindex 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 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.

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.
@xref{lzip-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.


@node Basic concepts
@chapter Basic concepts
@cindex basic concepts

@table @asis
@item Block
Any amount of data. A block is described by its starting position and
its size.

@item Cluster
Group of consecutive sectors read or written in one go.

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

@item 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.

@item Partition
Every part in which a device is divided. A partition normally contains a
filesystem. /dev/hda1, /dev/sdb3, are partition names.

@item 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

@item Sector
Hardware block. Smallest accessible amount of data on a device.

@end table


@node Algorithm
@chapter Algorithm
@cindex algorithm

GNU ddrescue manages efficiently the status of the rescue in progress
and tries to rescue the good parts first, maximizing the amount of data
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 secuentially, 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
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.

@sp 1
Note that as ddrescue splits the failed blocks, making them smaller, the
total error size may diminish while the number of errors increases.

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:

@noindent
Rescue the most important part of the disc first.
@example
ddrescue -i0 -s50M /dev/hdc hdimage logfile
ddrescue -i0 -s1M -d -r3 /dev/hdc hdimage logfile
@end example

@noindent
Then rescue some key disc areas.
@example
ddrescue -i30G -s10G /dev/hdc hdimage logfile
ddrescue -i230G -s5G /dev/hdc hdimage logfile
@end example

@noindent
Now rescue the rest (does not recopy what is already done).
@example
ddrescue /dev/hdc hdimage logfile
ddrescue -d -r3 /dev/hdc hdimage logfile
@end example


@node Invoking ddrescue
@chapter Invoking ddrescue
@cindex invoking
@cindex options
@cindex usage
@cindex version

The format for running ddrescue is:

@example
ddrescue [@var{options}] @var{infile} @var{outfile} [@var{logfile}]
@end example

ddrescue supports the following options:

@table @samp
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of ddrescue on the standard output and exit.

@item -b @var{bytes}
@itemx --block-size=@var{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.

@item -B
@itemx --binary-prefixes
Show units with binary prefixes (powers of 1024).@*
SI prefixes (powers of 1000) are used by default. (See table below).

@item -c @var{sectors}
@itemx --cluster-size=@var{sectors}
Number of sectors to copy at a time. Defaults to @w{64KiB / sector_size}.
Try smaller values for slow drives. The number of sectors per track (18
or 9) is a good value for floppies.

@item -C
@itemx --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.

@item -d
@itemx --direct
Use direct disc access to read the input file, 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.

@item -D
@itemx --synchronous
Use synchronous writes for output file. (Issue a fsync call after every
write). May be useful when forcing the drive to remap its bad sectors.

@item -e [+]@var{n}
@itemx --max-errors=[+]@var{n}
Maximum number of error areas allowed before giving up. Defaults to
infinity. If @var{n} is preceded by @samp{+} the number refers to new
error areas found in this run, not counting those already annotated in
the logfile.

@item -E @var{bytes}
@itemx --max-error-rate=@var{bytes}
Maximum growth per second of error size, in bytes, allowed before giving
up. Defaults to infinity.

@item -f
@itemx --force
Force overwrite of output file. Needed when the output file is not a
regular file, but a device or partition.

@item -F @var{types}
@itemx --fill=@var{types}
Fill the output file blocks specified as any of @var{types} in the
logfile, with data read from the input file. @var{types} contains one or
more of the status characters defined in the chapter Logfile Structure
(@pxref{Logfile Structure}). See the chapter Fill Mode (@pxref{Fill
Mode}) for a complete description of the fill mode.

@item -g
@itemx --generate-logfile
Generate an approximate logfile from the input and output files of the
original rescue run. Note that you must keep the original offset between
@samp{--input-position} and @samp{--output-position} of the original
rescue run.

@item -i @var{pos}
@itemx --input-position=@var{pos}
Starting position in input file, in bytes. Defaults to 0. In fill mode
it refers to the original input file. See the chapter Fill Mode
(@pxref{Fill Mode}) for details.

@item -m @var{file}
@itemx --domain-logfile=@var{file}
Restrict the rescue domain to the blocks marked as finished in the
logfile @var{file}. This is useful if the destination drive fails during
the rescue.

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

@item -n
@itemx --no-split
Stop after the trimming pass. Avoids spending a lot of time trying to
rescue the most difficult parts of the file. This option overrides the
@samp{--max-retries} option.

@item -o @var{pos}
@itemx --output-position=@var{pos}
Starting position in output file, in bytes. Defaults to
@samp{--input-position}. The bytes below @var{pos} aren't touched if
they exist and truncation is not requested. Else they are set to 0.

@item -p
@itemx --preallocate
Preallocate space on disc for output file. 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
@samp{--input-position}, @samp{--output-position}, @samp{--max-size},
and @samp{--domain-logfile} options.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -r @var{n}
@itemx --max-retries=@var{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.

@item -R
@itemx --reverse
Reverse direction of all copy operations. This means the copy of the
non-tried blocks, splitting and retrying are run backwards, while
trimming is run forwards.

@item -s @var{bytes}
@itemx --max-size=@var{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 output file. So, for example, the
following command creates an output file 300 bytes long, but only writes
data on the last 200 bytes:

@example
ddrescue -i 100 -s 200 infile outfile logfile
@end example

@item -S
@itemx --sparse
Use sparse writes for output file. (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.

@item -t
@itemx --truncate
Truncate output file to zero size before writing to it. Only works for
regular files, not for drives or partitions.

@item -T
@itemx --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 @samp{--retrim} is also specified, mark all failed blocks
inside the rescue domain as non-tried.

@item -v
@itemx --verbose
Verbose mode.

@end table

@sp 1
Numbers given as arguments to options (positions, sizes) may be followed
by a multiplier and an optional @samp{B} for "byte".

Table of SI and binary prefixes (unit multipliers):

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


@node Logfile Structure
@chapter Logfile Structure
@cindex 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 @samp{#} 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 @samp{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:

@multitable {Character} {generating approximate logfile}
@item Character @tab Meaning
@item '?'       @tab copying non-tried blocks
@item '*'       @tab trimming non-trimmed blocks
@item '/'       @tab splitting non-split blocks
@item '-'       @tab retrying bad sectors
@item 'F'       @tab filling specified blocks
@item 'G'       @tab generating approximate logfile
@item '+'       @tab finished
@end multitable

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:

@multitable {Character} {failed block bad-sector(s)}
@item Character @tab Meaning
@item '?'       @tab non-tried block
@item '*'       @tab failed block non-trimmed
@item '/'       @tab failed block non-split
@item '-'       @tab failed block bad-sector(s)
@item '+'       @tab finished block
@end multitable

@noindent
And here is an example logfile:

@noindent
# Rescue Logfile. Created by GNU ddrescue version @value{VERSION}@*
# Command line: ddrescue /dev/fd0 fdimage logfile@*
# current_pos  current_status@*
0x00120000     ?@*
#      pos         size   status@*
@multitable {0x00000000} {0x00000000} {status}
@item 0x00000000 @tab 0x00117000 @tab +
@item 0x00117000 @tab 0x00000200 @tab -
@item 0x00117200 @tab 0x00001000 @tab /
@item 0x00118200 @tab 0x00007E00 @tab *
@item 0x00120000 @tab 0x00048000 @tab ?
@end multitable

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


@node Examples
@chapter A small tutorial with examples
@cindex 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 physicaly
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.

@sp 1
@noindent
Example 1: Rescue a whole disc with two ext2 partitions in /dev/hda to
/dev/hdb.@*
Note: you do not need to partition /dev/hdb beforehand, but if the
partition table on /dev/hda is damaged, you'll need to recreate it
somehow on /dev/hdb.

@example
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
@end example

@sp 1
@noindent
Example 2: Rescue an ext2 partition in /dev/hda2 to /dev/hdb2.@*
Note: you need to create the hdb2 partition with fdisk first. hdb2
should be of appropiate type and size.

@example
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)
@end example

@sp 1
@noindent
Example 3: Rescue a CD-ROM in /dev/cdrom.

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

@sp 1
@noindent
Example 4: Rescue a CD-ROM in /dev/cdrom from two copies.

@example
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)
@end example

@anchor{lzip-example}
@sp 1
@noindent
Example 5: Rescue a lzip compressed backup from two copies on CD-ROM
with error-checked merging of copies (see the lzip manual for details
about lziprecover).

@example
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 --merge -o rescued.tar.lz rescued1.tar.lz rescued2.tar.lz
@end example

@sp 1
@noindent
Example 6: While rescuing the whole drive /dev/hda to /dev/hdb, /dev/hdb
fails and you have to rescue data to a third drive, /dev/hdc.

@example
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
@end example

@sp 1
@noindent
Example 7: While rescuing the whole drive /dev/hda to /dev/hdb, /dev/hda
stops responding and disappears from /dev.

@example
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 -T /dev/hda /dev/hdb logfile
ddrescue -d -f -r3 /dev/hda /dev/hdb logfile
@end example


@node Direct Disc Access
@chapter Direct Disc Access
@cindex direct disc access
@cindex raw devices

If you notice that the sizes and offsets 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 @samp{--block-size}
option for this to work. Try the @samp{--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 a explicit @samp{--max-size} or @samp{--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.

@sp 1
@noindent
Example 1: using direct disc access.

@example
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
@end example

@sp 1
@noindent
Example 2: using a raw device.

@example
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
@end example


@node Fill Mode
@chapter Fill Mode
@cindex Fill Mode

When ddrescue is invoked with the @samp{--fill} option it operates in
"fill mode", which is different from the default "rescue mode". That is,
if you use the @samp{--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 @samp{--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 needed data will be read.

Note that in fill mode the input file is always read from position 0. If
you specify a @samp{--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 @samp{--input-position} and @samp{--output-position} to
identical values, whereas when filling the output file of the original
rescue run you should keep the original offset between
@samp{--input-position} and @samp{--output-position}.

The @samp{--fill} option implies the @samp{--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.

@sp 1
@noindent
The fill mode has a number of uses. See the following examples:

@noindent
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 @samp{-} (bad-sector) with copies of the string
@w{@samp{BAD SECTOR }}:

@example
echo -n "BAD SECTOR " > tmpfile
ddrescue --fill=- tmpfile outfile logfile
@end example

@noindent
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.

@example
ddrescue --fill=+ --force /dev/zero bad_drive logfile
@end example

@noindent
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.

@example
ddrescue --fill=- --force --synchronous /dev/zero bad_drive logfile
@end example

@sp 1
@noindent
Fill mode can also help you to figure out, independently of the file
system used, what files are partially or entirely in the bad areas of
the disc. Just follow these steps:

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

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

3) Compute a md5sum or other checksum for every file. Build a list of
all the files and their checksums.

4) Fill the bad-sector blocks of the copied drive or image file with a
byte value different from zero.

5) Verify the checksums. Those files which have different checksums this
time reside (at least partially) in damaged disc areas.

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


@node Generate Mode
@chapter Generate-logfile Mode
@cindex 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 @w{@samp{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 @samp{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:
@example
ddrescue infile outfile
or
dd if=infile of=outfile conv=noerror,sync
@end example

you can generate an approximate logfile with this command:
@example
ddrescue --generate-logfile infile outfile logfile
@end example


@node Problems
@chapter Reporting Bugs
@cindex bugs
@cindex getting help

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
@email{bug-ddrescue@@gnu.org}. Include the version number, which you can
find by running @w{@samp{ddrescue --version}}.


@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye
