Ubuntu Manpage: myrescue - Harddisc Rescue

name
synopsis
description
note
options
recommended procedure
known bugs
authors
see also

NAME

       myrescue - Harddisc Rescue

SYNOPSIS

       myrescue  [-b  block-size]  [-B bitmap-file] [-A] [-S] [-r retry-count] [-f skip-failed] [-s start-block]
       [-e end-block] [-R] [-G good-range] [-F failed-range] [-T] [-J  jump-after-blocks]  [-U  usb-device-file]
       input-file output-file

DESCRIPTION

myrescue is a program to rescue the still-readable data from a damaged harddisk. It is similar in purpose
to dd_rescue, but it can be run in multiple passes and has options to avoid damaged areas to first handle
the not yet damaged part of the disk.

The program tries to copy the device blockwise to a file and keeps a table ("block bitmap") noting
whether a block has been successfully copied, not yet handled or has had errors. This block bitmap is
used in successive passes to only read the not yet rescued blocks.

The program has a special skip mode to handle read errors. Usually harddisk surface defects cover more
than just one block and continuous reading in defect areas can damage the surface, the heads and (by
permanent recalibration) the drive mechanics. If this happens, the chances of rescuing the remaining
undamaged data drop dramatically. So in skip mode, myrescue tries to get out of damaged areas quickly by
exponentially increasing the stepsize. The skipped blocks are marked as unhandled in the block bitmap and
can be retried later.

As another alternative, the program can jump around on the disc, trying random blocks to first get an
overview of the damage. In case of regular defects (e.g. an entire failed head) this can be used to
determine the physical disc structure and to avoid these regions in the first run.

Also there are options to avoid getting close to already recognized defects or stay in the proximity of
good regions.

Finally, the program has an option to multiply try to read a block before considering it damaged.

NOTE

       This tools is no replacement for a professional data recovery service!  If you do have the latter option,
       don't even think of using myrescue, as it may further damage your disk. This tool is  provided  only  for
       the  case that you are absolutely desperate and definitely cannot afford a professional data recovery and
       know what you are doing.

       Data recovery - whether  professional  or  DIY  -  is  always  careful  detective  work.  Damaged  drives
       deteriorate  rapidly  and  behave  nondeterministically,  you may not get a second chance once you made a
       mistake. So you have to be fully aware of what you're doing and understand precisely what's happening. If
       you are unsure it's probably best to stop right now and ask a linux guru for assistance.

       In any case do not expect too much. While complete restores have been witnessed, you should not take them
       for granted. A better attitude is to consider your data lost and be glad for any survivors that turn up.

       The usual GPL disclaimer applies. Especially the NON-WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE.  Don't
       blame (or sue) me if it fails to recover or further damages your data.

       And  a  final word you probably don't want to hear in this situation: For the future consider a routinely
       backup to avoid a "next time".

OPTIONS

-b block-size
The size of the blocks (in bytes). Set this to your harddiscs error detection/correction unit
size. Usually this is 4096, which happens to be the default.

-B bitmap-file
The file containing the status table of all blocks. Nice (or frightening...) to view with hexdump.
01 means OK; 02 means that the block was OK, but took a long time to read; 00 means not yet done;
negative values mean the number of failed read attempts. If not given, defaults to output-
file.bitmap

-A Abort when encountering errors.

-S Activate skip mode: When encountering errors increase the stepsize exponentially until a readable
block is found.

-f skip-failed
Skip blocks that have already had skip-failed failures. Useful to avoid scratching the same block
over and over again.

-r retry-count
The number of times to read a block before it is declared bad for this run. (You can still retry
it on the next run.) Default: 1

-s start-block
The number of the block to start with. Default: 0

-e end-block
The number of the block, where reading stops (not included!). Default: size of input-file divided
by block-size.

-R Reverse reading direction, i.e. from end-block (excluded) to start-block

-G good-range
Only try to read blocks within good-range blocks from an already successfully read block.

-F failed-range
Extends -f to also skip any block within failed-range blocks of a block to be skipped as specified
by -f.

-T Also avoid blocks, that were successfully read, but took an unusually long time to read. In case
of growing defects, these blocks can mark the boundaries of defective regions. This options makes
-A, -S and -F treat such blocks as defective.

-J jump-after-blocks
Randomly jump across the disc after reading jump-after-blocks blocks in both directions. This
might be useful to scan discs with scattered defects. In jump mode -S causes myrescue to jump to
a new block upon the first failed sector or upon hitting a sector to be skipped as specified by
-f, -G or -F.

-U usb-device-file
(Linux-specific feature) Perform a bus reset for the specified usb-device-file after every failed
read attempt. Certain faulty SD cards will disappear completely from the bus after a read error
occurs and the USB reader has to be reset completely in order to continue.

-h, -? Display usage information.

RECOMMENDED PROCEDURE

NOTE: In data recovery every case needs special consideration and a specific approach - mindlessly
running procedures is a bad idea. So consider the following an example, keep a close eye on the process
and be prepared to stop and reconsider if anything unusual happens. And once again: If you are feeling
unsure, stop and ask someone experienced for assistance.

• Make sure you have sufficient disk space for twice the whole partition (complete partition size,
not just the used amount of data) plus some space for the block bitmap (1 byte per block). Of
course this should NOT be on the damaged disc.

• Determine the hardware block size (CRC/ECC unit) of your harddisk. This may be found out from
hdparm, some entries in /proc/ide/hd? or on the web. I have not yet checked whether this is
possible with an ioctl. If you have, please let me know.

• Start a skip mode run with one retry per block to first copy the undamaged area.

• Start a normal run with one retry per block to copy the remaining skipped blocks. You may try to
use -f 1 to skip the damaged blocks from the first run.

• Repeat until the number of errors seems to have converged. Try waiting a couple of hours between
the retries.

• Repeat this with higher retry counts and wait for convergence.

• Make a copy of the rescued data and run fsck on it. The copy is important! Running fsck is risky
if blocks containing filesystem structures (superblocks, directories, inodes, journal, ...) are
missing. Sometimes the well-meant attempt to correct the filesystem can damage the image file, so
always work on a copy.

• Mount the filesystem (if copied to a file: via loopback) and check your data. If directory
information has been destroyed, fsck moves unidentifiable file fragments to lost+found, so you
should also check this location.

It may help to try reading non-defect areas in between to allow the drive to recalibrate.

The developers are glad to hear about your experiences. Please post them to the Experiences forum on the
Sourceforge Project page. Thank you!

KNOWN BUGS

       The  handling  of  the  bitmap-file  currently relies on the filesystem semantics, that when lseek(2) ing
       beyond the end of file and then writing, the space in between is filled with zero-bytes.

       The block bitmap maxes out after 127 failed read attempts.

AUTHORS

       Kristof Koehler <kristofk@users.sourceforge.net>, Peter Schlaile <schlaile@users.sourceforge.net>