Friday, March 15, 2024

Cloning Drives - Data Recovery with Open-Source Tools (part 5)

This is part 5 of a multi-part series. See part 1 for the beginning of the series.

Cloning hard drives with dd_rescue

In cases where a hard drive is failing, often simply cloning the drive is all that is required to recover data. There are many other situations where cloning a drive is important though, such as when attempting to recover from a broken partition table or major filesystem corruption.

The primary tool for cloning drives is called dd_rescue. Running dd_rescue -h or simply dd_rescue with no options will give you a summary of the various command-line options:

dd_rescue Version 1.14,, GNU GPL
 ($Id: dd_rescue.c,v 1.59 2007/08/26 13:42:44 garloff Exp $)
dd_rescue copies data from one file (or block device) to another.
USAGE: dd_rescue [options] infile outfile
Options: -s ipos start position in input file (default=0),
	     -S opos start position in output file (def=ipos),
	     -b softbs block size for copy operation (def=65536),
	     -B hardbs fallback block size in case of errs (def=512),
	     -e maxerr exit after maxerr errors (def=0=infinite),
	     -m maxxfer maximum amount of data to be transfered (def=0=inf),
	     -y syncfrq frequency of fsync calls on outfile (def=512*softbs),
	     -l logfile name of a file to log errors and summary to (def=""),
	     -o bbfile name of a file to log bad blocks numbers (def=""),
	     -r reverse direction copy (def=forward),
	     -t truncate output file (def=no),
	     -d/D use O_DIRECT for input/output (def=no),
	     -w abort on Write errors (def=no),
	     -a spArse file writing (def=no),
	     -A Always write blocks, zeroed if err (def=no),
	     -i interactive: ask before overwriting data (def=no),
	     -f force: skip some sanity checks (def=no),
	     -p preserve: preserve ownership / perms (def=no),
	     -q quiet operation,
	     -v verbose operation,
	     -V display version and exit,
	     -h display this help and exit.
Note: Sizes may be given in units b(=512), k(=1024), M(=1024^2) or G(1024^3) bytes
This program is useful to rescue data in case of I/O errors, because
 it does not necessarily abort or truncate the output.

Note that there is also a GNU ddrescue with a similar feature set, but with entirely incompatible command-line arguments.

In the simplest of cases, dd_rescue can be used to copy infile (let's say, for example, /dev/sda) to outfile (again, for example, /dev/sdb).

dd_rescue /dev/sda /dev/sdb

In most cases, you'll want a little more control over how dd_rescue behaves though. For example, to clone failing /dev/sda to /dev/sdb:

dd_rescue -d -D -B 4k /dev/sda /dev/sdb

(to use the default 64k block size) or, for really bad drives, to force only one read attempt:

dd_rescue -d -D -B 4k -b 4k /dev/sda /dev/sdb

Adding the -r option to read backwards also helps sometimes.

Changing block sizes

By default, dd_rescue uses a block size of 64k (overridden with -b). In the event of a read error, it tries to read again in 512-byte chunks (overridden with -B). If a drive is good (or only beginning to fail), a larger block size (usually in the 512kB-1MB range) will give you significantly better performance.

If a drive is failing, forcing the default block size to the same value as the fall-back size will keep dd_rescue from re-reading (and therefore possibly damaging) failed blocks.

Direct I/O

The -d and -D options turn on direct I/O for the input and output files respectively. Direct I/O turns off all OS caching, both read-ahead and write-behind. This is much more efficient (and safer) when reading from and writing to hard drives, but should generally be avoided when using regular files.

Other useful options

-r        Read backwards. Sometimes works more reliably. (Very handy trick...)

-s num    Start position in input file.

-S num    Start position in output file. (Defaults to the same as -s.)

-e num    Stop after num errors.

-m num    Maximum amount of data to read.

-l file   Write a log to file.

Copying partitions

Let's say you have a drive with a MS-DOS partition table.  The drive has two partitions.  The first is a NTFS partition that seems to be intact.  The second partition is an unknown type.  Rather than copying every block using dd_rescue, you want to copy only the blocks that are in use to a drive that is the same size.

To do this, first copy the boot sector and partition table from /dev/sda to /dev/sdb using dd:

dd if=/dev/sda of=/dev/sdb count=1

The default block size of dd is 512 bytes, which, conveniently, is the size of boot sector + partition table at the beginning of the drive.

Note: This trick doesn't quite work on MS-DOS partition tables with extended partitions! In that case, use sfdisk to copy the partition table (after running the above command to pick up the boot sector):

sfdisk -d /dev/sda | sfdisk /dev/sdb

Next, re-read the partition table on /dev/sdb using hdparm:

hdparm -z /dev/sdb

Next we can clone the NTFS filesystem on /dev/sda1 to /dev/sdb1 using the ntfsclone command from ntfsprogs:

ntfsclone --rescue -O /dev/sdb1 /dev/sda1

Finally we clone /dev/sda2 to /dev/sdb2 using dd_rescue using a 1MB block size (for speed):

dd_rescue -d -D -B 4k -b 1M /dev/sda2 /dev/sdb2

To be continued in part 6.

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