Optical media might have lost their prominence to other portable storage mediums like USB pen drives, but they still have their uses. DVDs figure in my backup rotation policy for housing files that are over a year old. They are also a good medium to store a bare-metal image of a fully-setup machine.
But optical media fails more often than one would hope. One way around this issue is to make multiple copies of the backup media. However, making multiple copies of backup disks seems like an inefficient use of resources.
A much better option is to use the DVDisaster utility. The tool creates an Error Correction Code (ECC) file from a healthy disk, which can be used later to recover data when the medium is damaged.
Note: DVDisaster is available for FreeBSD, GNU/Linux, NetBSD and Windows. This tutorial is done on Linux, but the steps are the same for every OS.
DVDisaster works on ISO images. To image your disk, insert the disc into the drive, and launch DVDisaster after the disk has spun down. Now click on the “Image file selection” icon and type in a name for the ISO image and select a directory for it to be stored in, and click the “Read” button. The app will read the disk sector by sector, and create the image as per the name and location you specified earlier.
Before you generate the ECC file for this image, note that DVDisaster supports two types - RS01 and RS02. The former method stores the ECC file in a remote location, while the latter bundles it along with the ISO image.
To make your selection, click on the tool icon to launch the app’s Preferences menu and switch to the “Error correction” tab. Here select the storage method from the pulldown menu. I’d advise you to stick to the default RS01 method and store the ECC file on a separate medium.
To create the ECC file, click on the “Error correction file selection” icon, type in a name for the ECC file, and click on the “Create” button. DVDisaster now reads the ISO and create its ECC file. Using the default settings, the ECC file is about 15% of the size of the ISO image.
For better protection, head back to the “Error correction” tab under the Preferences menu and switch to the “High” option. Your ECC file will now be about 35% the size of the disc, but will give you a better chance to restore a badly damaged media. Using the other options, you can also manually specify the level of redundancy or restrict the ECC file to a specified size.
Restore damaged disks
It’s a good idea to regularly check the backup media with DVDisaster. Just insert the media in the drive, and click on the “Scan” button. If the scan detects bad sectors in the media, it’s time to recover the lost data.
To do so, you first need to create an ISO image of the damaged media. The procedure is the same as above – specify an ISO image name, and use the “Read” button.
Now locate the ECC file you created earlier for the damaged media, and point to it using the “Error correction file selection” button. With the image and ECC file in place, click on the “Fix” button which reads and repairs the damaged image.
The success rate of the recovery depends on the state of the damaged disk, which is why it is necessary to scan the media regularly, and repair it as soon as bad blocks show up.
The entire process might sound daunting initially, but once you’ve run through it a couple of times, it’ll become second nature. With the cost of media in a constant freefall, making multiple backup copies sounds like an easier option - if one goes kaput, you have another one to fallback to. But eventually all copies will go bad, and you”re back to square one. Which is why it makes sense to invest a little time and effort mastering DVDisaster.