The big picture

dvdisaster Version 0.72.6 / 0.79 (devel-3) | To the Internet version

Contents
News
Overview
Howtos
· | The big picture
| - | error correction
| - | correct usage
| - | incorrect usage
· | Scanning media for errors
· | 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
|  | ### The big picture - a comparison of dvdisaster with conventional backup dvdisaster stores data on CD/DVD/BD in a way that the data is fully recoverable even after the medium has developed some read errors. The method employed in dvdisaster uses less storage space (or additional media) than a full backup would do. Before using dvdisaster it is important to understand the similarities and differences between dvdisaster and a conventional (full) backup: Let's first consider how a conventional backup scheme works:
| Copy |  |  | An existing medium (1) is copied onto a backup medium (2).
|  |  |  | If any one of the two media is damaged afterwards, we still have an intact medium left.
There are actually some cases where it is important to keep a second copy of a CD/DVD/BD: One medium might get lost, burst while spinning in the drive, or it may be destroyed due to mishandling. However such cases of complete data loss are rare as long as media are handled properly. It is more likely that the medium starts to gradually lose data after a few years - a nearly unavoidable aging process. When the medium is regularly used (or scanned for defects) the data loss will typically be noticed after 5% to 10% of the medium have already become unreadable. At this point the medium is unusable as a whole, but maybe 90% of it is still readable. On the other hand a full backup copy of the medium is not required; we simply need a method for recovering the missing 10% of data. This is where dvdisaster comes into play. Consider this:
| Create ECC |  |  | This time we do not make a full backup. dvdisaster is used to create error correction data ("ECC") which can recover up to 20% of a degraded medium. The value of 20% was chosen to have a safety margin over the expected data loss of 5-10%.
|  |  |  | Wenn the medium fails at a later time, its contents are recovered from its still readable parts and from the error correction data.
80% |  | 20% |  | For a successful recovery at least 80% of the data must still be readable from the medium, and the remaining 20% are recalculated from the error correction data.
|  |  |  | The completely recovered data is now available as an ISO image on the hard drive (the medium remains defective as physical data loss is irrevocable).
|  |  |  | Write the image to a blank medium using your favourite CD/DVD/BD authoring software.
|  |  |  | You now have a new error-free medium.
As you have seen the data recovery took more steps then doing a conventional backup. So let's summarize the pros and cons of dvdisaster compared with conventional backup:
Advantages | dvdisaster uses less storage. When using error correction data with a 20% recovery capability, protecting 5 media requires only one additional medium for the ECC data. Since all media will eventually age and start losing data in similar places (typically in the outermost region), doing a 1:1 copy might not help at all. Both copies may turn out defective in the same places after a few years.
Similarities | Both backup copies and error correction data must be created before the master disc fails. You can't create them from an already defective medium.
Disadvantages | If the recovery capability of the error correction data is exceeded (or the medium gets lost), no data can be recovered! Especially take note that error correction data with a repair rate of 20% together with a 75% readable the medium does not result in 95% recovery! In that case, nothing beyond the 75% readable data from the medium can be recovered!
The next three pages provide more related information: The general idea of the error correction is explained. Jane demonstrates the proper usage of dvdisaster. She will create error correction data in advance and is therefore able to recover all data when her media become defective. However you should not follow the way of Joe. He does not use error correction data and finds out that his defective media are not recoverable even after multiple reading passes. As a consequence he loses data from a defective medium. Of course these stories are purely fictional and any similarities with existing persons or situations are purely conincidental.

Copyright 2004-2014 Carsten Gnörlich. Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.