Optical storage media enjoy great popularity and are used above all in multimedia applications and often as backup media. However, these cheap plastic discs also have disadvantages: Aging processes cause read errors over time, ultimately making the entire medium unusable and leading to complete data loss in extreme cases.

Predicting when such problems will arise is difficult; it depends not least on environmental influences and the original quality of the medium. If you want to use optical media for long-term archiving, you are advised to exercise due diligence. Using a small program named Dvdisaster [1] can effectively help prevent loss of data on optical storage media.

All the major distributions have Dvdisaster in their repositories; you can typically avoid building from the source code [2] and install the program easily using your distribution's package manager. Depending on the desktop used, you will then find a starter for Dvdisaster below Multimedia or System Tools. Note that if you use a Blu-ray burner, you need a Dvdisaster version from the current 0.72 branch; the predecessors do not support Blu-ray media.

Operations

Dvdisaster reads the contents of the storage medium sector by sector and therefore operates independently of the specified filesystem. In this way, the software reconstructs damaged content on optical storage media solely by reference to the error correction data created previously. Because data recovery generally even fails with a partially unreadable optical disc, it is advisable to use the software while the CD or DVD is still working correctly. Dvdisaster also provides some routines that analyze the state of an optical medium. The program includes error correction data – if present – in the scan. As an alternative for this function, you can also use the free QPxTool [3].

The error correction data generated by Dvdisaster is not a complete backup copy of the original medium; it can reconstruct a maximum of about 20 percent of the original dataset. From this error correction data, and the still-readable content of the disc, the software then creates a new ISO image that can be burned to a new optical medium. Alternatively, you can save the ISO image on another storage medium and burn it later if necessary. However, you do not need to recreate the error correction data every time you burn the ISO image; it can be reused for testing and for recovery purposes for later burning operations, too.

Ready, Steady, Scan

After completing the installation, a somewhat unusual-looking interface appears when you first launch: The controls are on the right-hand side of the screen in the form of buttons, and the menubar consisting solely of the File and Tools entries.

To avoid later unpleasant surprises, it makes sense first to check the integrity of the volume you are backing up before creating an image. To do this, insert the original storage medium and click the Scan button to the right of the program window. The software then reads the disc from the inside outward and shows its progress as a spiral under the Medium state section.

The graph on the left-hand side of the window shows the read speed. If you see dips in what is normally a linearly rising curve, these are indicative of problematic sectors. Dvdisaster also shows the unreadable sectors of an optical disc in red in the sector display on the right in the area below Medium state, whereas unproblematic data areas are marked in green (Figure 1). For checksum errors, Dvdisaster highlights the corresponding sectors in yellow.

Figure 1: If the DVD is almost new, Dvdisaster should not report any errors on checking the data carrier.

At the top of the program window, you will find the model name of the active DVD drive with additional path information for the ISO image and error correction file to be created. Unless you change the paths, Dvdisaster will store both files in your home directory.

If Dvdisaster marks many sectors in yellow or red during reading of an optical medium, then you can review detailed information for the storage medium in the Tools | Medium info menu item. Additionally, the program gives you a detailed status report when you click the View log button at the bottom right in the program window.

Please note that, although Dvdisaster theoretically supports all optical discs, this does not include industrially produced media. If you feed a commercial audio or video CD, DVD, DVD-ROM, or Blu-ray disc to Dvdisaster, the software immediately tells you that it cannot process that type of medium.

If you want to edit double-layer DVDs with twice the storage capacity in Dvdisaster, your drive needs to support this kind of medium. Although this is the case for almost all currently available DVD burners, it is not necessarily true for older models and combination drives.

Correction Data

For best results when creating the error correction data, you should always adjust Dvdisaster's operating mode to suit the technical requirements. To access the detailed settings dialog, click the stylized wrench at the top right in the program window. This opens a Preferences window that groups various settings in seven horizontally arranged tabs.

The file containing the error correction data uses either an optical storage medium or an ISO image as its basis. You will thus want to use the ISO/UDF setting in the Get image size from section, and Linear as the Reading strategy. Additional changes are made in the Read attempts and Error correction tabs. If you are using an optical data carrier, you can go to the Read attempts section and enable the Read and analyze raw sectors switch to enable verification of the data that is read against additional parameters supplied by the drive (Figure 2).

Figure 2: Clear-cut Preferences tabs allow users to set up Dvdisaster to best suit their needs.

You can then go to the Error correction tab and configure the data redundancy settings. Because the volume of correction data increases in parallel with the number of reconstructable read errors, you can adjust the redundancy values to define whether a severely damaged image can still be read.

The default Normal is equivalent to a redundancy of about 14 percent, and the High setting creates around 33 percent correction data. The Other slide control lets you to define a percentage manually. If you do not want the error correction file to exceed a certain size, then enter the value in the Use at most field in megabytes (Figure 3).

Figure 3: Parameters for the error correction file are defined in the Error correction tab.

After completing the basic settings, you can start creating an image by pressing the Read button. Dvdisaster then reads the optical data carrier and creates an ISO image. To create an error correction file, press Create after creating the ISO image. The program visualizes the course of this action with a progress indicator bar (Figure 4).

Figure 4: The error correction file is created in a separate step.

If you are using an ISO image rather than an optical disc for the error correction file, enter the file name for the ISO image in the input box at top center in the program window. Then, press the Create button to move directly to creating an ECC file with error correction data.