Interpreting the results

dvdisaster Version 0.72.4 / 0.72 (pl4) | To the Internet version

Contents
News
Overview
Howtos
· | The big picture
· | Scanning media for errors
| - | Basic settings
| - | Performing the test
| - | Interpreting the results
| - | Advanced settings
· | Putting error correction data on a separate file
· | Putting error correction data on the medium
· | Recovering media images
· | Getting information about images and error correction data
· | Testing image compatibility
· | Dialogs and buttons
Download
Questions and Answers
Bug reporting
Scanning media for errors |
Interpreting results
Overview. dvdisaster provides several information about the scanning results: The spiral under "Medium State" (to the right). The spiral provides information about the medium readability. The medium is fully readable when all segments of the spiral are colored green. Yellow or red blocks mark places where data could not be correctly read from the medium. The total number of unreadable sectors is printed in the "Scanning finished:" message at the window bottom. "Speed" - The reading speed curve (upper left). The reading speed is not an absolute gauge of the medium health, but it is usable as a rule of thumb: The more regular the curve, the better the medium. You will find examples of good and bad reading speed curves further down this page. "C2 errors" - A medium state gauge provided by the drive (down left). This kind of analysis is currently only available for CD media. CD drives have a built-in error correction which can eliminate small data losses caused by minor defects on the medium. The number of C2 errors is a measurement of how often the drive needed to employ its internal error correction during the read - this value should be zero on good media. |
Overview
Examples for good media
Good CD: This screen shot shows a perfect CD: All blocks under "Medium state" are green, no C2 errors have been reported and the reading curve runs smoothly. A rising reading speed is normal for most media (see the next screen shot for a counter example). The small spikes at the beginning and at the end of the curve are normal; minor glitches like the one shown at 250M are also harmless. |
Good CD
Sometimes the reading curve won't rise steadily: Multi-layered media might yield reading curves which are rising and dropping in a symmetric pattern. Not shown but also possible are flat curves without any change in reading speed (most typically seen with DVD-RAM). |
Good two layered DVD
An example for a weak medium
This medium is still readable as indicated by the green spiral shown under "Medium state". However there are clear signs of serious trouble ahead: The drive must slow down significantly towards the end of the medium in order to read from it. Note the steep fall of reading speed after the 600M mark. This comes along with C2 error rates rising to the 100 mark; this is another warning that the medium is decaying in the outer region. If you have not created error correction data this is probably the last opportunity to do so as the medium will develop the first read errors soon.
Screen shot: Weak CD
Examples of defective media
Defective CD. The red sectors in the spiral visualize large unreadable sections in the outer region of the medium. At the bottom of the window you will find the information that the medium contains 28752 unreadable sectors. This sums up to about 8.2% defective sectors (of 352486 sectors total) and is well within the recovery bounds by error correction (ecc) data made with default settings - if you have made the ecc data in time! Otherwise the contents of the red sectors are lost since ecc data cannot be created from already defective media. |
Defective CD
Checksum errors. Yellow spots in the spiral depict places where the medium was fully readable, but the data read did not match checksums in the error correction data. There are two main causes: The image has been manipulated after the creation of error correction data and before writing it to the medium. This can happen on Unix systems when the image is mounted with write access after ecc data has been created. Typical signs are CRC errors in sector 64 and in sectors 200 to 400 as the system updates the file access times there. Performing a data recovery using dvdisaster is typically harmless in this situation. However if you have modified files in the image after creating the ecc data, the error correction data will be both worthless and dangerous. Applying a recovery to the medium will restore the image state at the time the ecc data has been created, and this will obviously not represent the most recent contents of the medium. There are technical problems with the computer system, especially in mass storage communication. Perform the scan again and observe the CRC error locations. If CRC errors disappear or surface at different locations your system might have defective RAM, bad drive cabling/controllers or incorrect clock speeds. |
Checksum errors

Copyright 2004-2012 Carsten Gnörlich. Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.