Difference between pages "TDMA" and "Ddrescue"

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m (Why use TDMA?)
 
(Added a warning line to the examples section clarifying something that caused me to overwrite an entire disk by accident and lose a lot of data in the process.)
 
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{{Wikify}}
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{{Infobox_Software |
 +
  name = ddrescure |
 +
  maintainer = [[Antonio Diaz Diaz]]|
 +
  os = {{Linux}}|
 +
  genre = {{Disk imaging}} |
 +
  license = {{GPL}} |
 +
  website = [http://www.gnu.org/software/ddrescue/ddrescue.html http://www.gnu.org/software/ddrescue/ddrescue.html] |
 +
}}
  
'''TDMA - Time Division Multiple Access'''
+
'''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.
  
== Why use TDMA?==
+
From the [[ddrescue]] info pages:
TDMA (Time Division Multiple Access), is used in the largest available networks in the world. It is a digital communication method allowing many users to access a single communication channel.  TDMA is aimed at dealing with multiple access to the same communication medium.  Each individual user is given a unique time slot within the defined communication channel.  This methodology increases the efficiency of transmission by allowing multiple users simultaneous access to a time slot.  A
+
<blockquote>
significant benefit is TDMA can be easily adapted to transmission of data as well as voice communication.  
+
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.<br><br>
  
TDMA offers the ability to carry date rates of 64 kbps to 120 MBPS, which enables options of communication such as fax, voiceband data, sms, as well as bandwidth intensive apps. TDMA allows the mobile device to have extended battery life, since the cellular device is only transmitting a portion of the time during conversations.  In addition, TDMA is the most cost effective technology for upgrading an analog system to digital.
+
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.<br><br>
  
==How it works==
+
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.<br><br>
  
It’s necessary for TDMA to rely upon that fact that the audio signal has been digitized.  These signals are divided into a number of milliseconds.  TDMA is also the access technique used in the European digital standard, GSM, and the Japanese digital standard, personal digital cellular (PDC). The reason for choosing TDMA for all these standards was that it enables some vital features for system operation in an advanced cellular or PCS environment. Today, TDMA is an available, well-proven technique in commercial operation in many systems.
+
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.<br><br>
A single channel can carry all four conversations if each conversation is divided into relatively short fragments, is assigned a time slot, and is transmitted in synchronized timed.
+
  
==Pros & Downfaults==
+
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.
 +
</blockquote>
  
TDMA can be wasteful of bandwidth because time slots are allocated to specific conversations whether or not anyone was speaking at the given moment.  There is an enhanced version however, EDTMA, which attempts to correct this problem.  Unlike TDMA which waits to determine whether a subscriber is transmitting, ETDMA assigns subscribers using a dynamic method.  The data is sent through pauses which normal speech contains.  If the subscriber has something they would like to transmit, it is placed as one bit in the buffer queue.  The system then scans the buffer and notices the user has something to transmit, allocating the bandwidth accordingly.  However, if there is nothing to transmit, it goes to the next subscriber.  This technique can be 10 times more efficient as analog transmission of TDMA. 
+
== Installation ==
  
==Cell Phone Providers==
+
=== Bootable CD ===
 +
ddrescue is available on bootable rescue cds such as SystemRescueCd http://www.sysresccd.org/Main_Page.
 +
=== 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.
  
There are several cell phone companies competing to sell their phones and advertise their network coverage area.  They will primarily be competing within two categories:  TDMA and [[CDMA]].  The pros and cons of TDMA have been mentioned, and the companies that offer TDMA are the following:  AT&T, Cingular, Nextel, T-Mobile.  The companies that support [[CDMA]] are:  ALLTEL, Amp'd Mobile, Cricket Wireless, ESPN, Quest, Sprint, Verizon, Virgin Mobile.  As we can see, more companies are supporting [[CDMA]].  The question that arises is, what makes the two so different? 
+
Debian
 +
<blockquote>
 +
aptitude install gddrescue
 +
</blockquote>
 +
Ubuntu
 +
<blockquote>
 +
sudo apt-get install gddrescue
 +
</blockquote>
 +
=== Gentoo ===
 +
<blockquote>
 +
emerge ddrescue
 +
</blockquote>
 +
== 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.
  
==TDMA Vs. [[CDMA]]==
+
First you copy as much data as possible, without retrying or splitting sectors:
 +
<blockquote>
 +
ddrescue --no-split /dev/hda1 imagefile logfile
 +
</blockquote>
  
TDMA is better for international plans and debately has better battery life.  [[CDMA]] claims it has better battery life and coverage, however, Cingular is supposed to have the best coverage area and that is TDMA.  All of this started however ever since [[CDMA]] was introduced in 1989, and the wireless world has been in debate over merits of TDMA and [[CDMA]].  Those who are for [[CDMA]] have claimed that its technology has bandwidth efficiency of up to 13 times that of TDMA and between 20 to 40 times that of analog transmission.  Furthermore, [[CDMA]] lovers say its spread spectrum technology is more secure and offers higher transmission quality than TDMA because of TDMA's increased resistance to multipath distortion.   
+
Now let it retry previous errors 3 times, using uncached reads:
 +
<blockquote>
 +
ddrescue --direct --max-retries=3 /dev/hda1 imagefile logfile
 +
</blockquote>
  
Those who favor TDMA point out that there has been no successful major trial of [[CDMA]] technology that supports the capacities it claims.  Not to mention, theoretical improvements in bandwidth efficiency claimed [[CDMA]] is now being approached by enhancements to TDMA technology. TDMA's evolution allows capacity increases of 20 to 40 fold over analog in the near future.  [[CDMA]] is a very expensive technology that needs $300,000 per base station, compared to $80,000 for TDMA.  Lastly, TDMA is the proven leader as the most economical digital migration path for existing AMPS networks.  No one has the final word in this debate, however, it is evident that TDMA will remain the dominant technology in the wireless market.
+
If that fails you can try again but retrimmed, so it tries to reread full sectors:
 +
<blockquote>
 +
ddrescue --direct --retrim --max-retries=3 /dev/hda1 imagefile logfile
 +
</blockquote>
  
== External Links ==
+
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.
  
http://en.wikipedia.org/wiki/Time_division_multiple_access <br>
+
=== Before linux kernel 2.6.3 / 2.4.x ===
http://www.iec.org/online/tutorials/tdma/topic04.html <br>
+
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.
http://www.cellphoneinfo.com/index.html
+
 
 +
First you copy as much data as possible, without retrying or splitting sectors:
 +
<blockquote>
 +
ddrescue --no-split /dev/hda1 imagefile logfile
 +
</blockquote>
 +
 
 +
Now change over to raw device access. Let it retry previous errors 3 times, don't read past last block in logfile:
 +
<blockquote>
 +
modprobe raw<br>
 +
raw /dev/raw/raw1 /dev/hda1<br>
 +
ddrescue --max-retries=3 --complete-only /dev/raw/raw1 imagefile logfile
 +
</blockquote>
 +
 
 +
If that fails you can try again (still using raw) but retrimmed, so it tries to reread full sectors:
 +
<blockquote>
 +
ddrescue --retrim --max-retries=3 --complete-only /dev/raw/raw1 imagefile logfile
 +
</blockquote>
 +
 
 +
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:
 +
<blockquote>
 +
raw /dev/raw/raw1 0 0
 +
</blockquote>
 +
 
 +
== 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 drive you are copying to. If you do not want to do that, rather create an image of the disk to be rescued.'''
 +
<blockquote>
 +
ddrescue -r3 /dev/hda2 /dev/hdb2 logfile<br>
 +
e2fsck -v -f /dev/hdb2<br>
 +
mount -t ext2 -o ro /dev/hdb2 /mnt<br>
 +
</blockquote>
 +
 
 +
Example 2: Rescue a CD-ROM in /dev/cdrom
 +
<blockquote>
 +
ddrescue -b 2048 /dev/cdrom cdimage logfile
 +
</blockquote>
 +
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=<bytes>
 +
    hardware block size of input device [512]
 +
-B, --binary-prefixes
 +
    show binary multipliers in numbers [default SI]
 +
-c, --cluster-size=<blocks>
 +
    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=<n>
 +
    maximum number of error areas allowed
 +
-F, --fill=<types>
 +
    fill given type areas with infile data (?*/-+)
 +
-g, --generate-logfile
 +
    generate approximate logfile from partial copy
 +
-i, --input-position=<pos>
 +
    starting position in input file [0]
 +
-n, --no-split
 +
    do not try to split or retry error areas
 +
-o, --output-position=<pos>
 +
    starting position in output file [ipos]
 +
-q, --quiet
 +
    quiet operation
 +
-r, --max-retries=<n>
 +
    exit after given retries (-1=infinity) [0]
 +
-R, --retrim
 +
    mark all error areas as non-trimmed
 +
-s, --max-size=<bytes>
 +
    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]] [http://en.wikipedia.org/wiki/Out_of_the_box Out of the Box]. Precompiled packages are available in the [http://cygwin.com/packages/ Cygwin distribution]. This makes it usable natively on [[Windows]] systems.
 +
 
 +
== See also ==
 +
 
 +
* [[aimage]]
 +
* [[Blackbag]]
 +
* [[dcfldd]]
 +
* [[dd]]
 +
* [[dd_rescue]]
 +
* [[sdd]]

Revision as of 21:53, 14 November 2009

ddrescure
Maintainer: Antonio Diaz Diaz
OS: Linux
Genre: Disk imaging
License: GPL
Website: http://www.gnu.org/software/ddrescue/ddrescue.html

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.

Contents

Installation

Bootable CD

ddrescue is available on bootable rescue cds such as SystemRescueCd http://www.sysresccd.org/Main_Page.

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 drive you are copying to. If you do not want to do that, rather create an image of the disk 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=<bytes>
   hardware block size of input device [512] 
-B, --binary-prefixes
   show binary multipliers in numbers [default SI] 
-c, --cluster-size=<blocks>
   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=<n>
   maximum number of error areas allowed 
-F, --fill=<types>
   fill given type areas with infile data (?*/-+) 
-g, --generate-logfile
   generate approximate logfile from partial copy 
-i, --input-position=<pos>
   starting position in input file [0] 
-n, --no-split
   do not try to split or retry error areas 
-o, --output-position=<pos>
   starting position in output file [ipos] 
-q, --quiet
   quiet operation 
-r, --max-retries=<n>
   exit after given retries (-1=infinity) [0] 
-R, --retrim
   mark all error areas as non-trimmed 
-s, --max-size=<bytes>
   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