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Comment by Luc | Hacker News Reader

Cool, but the method of verifying the data (playing back the movie) seems non-optimal. The movie could have had some data corruption that went unnoticed.

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Ideally the test should include the number of bit errors that were corrected using on-disc ECC. This could then also be used to estimate disc lifetime (preferably using multiple samples).

I have a script that creates a hash based on all files in a directory - photos 2004. Then save the hash separately to a text file.

I have 3 copies so I can check the archive version, active storage volume, and local version to see if any lost integrity in the transfer process.

I’m curious how it would compare against my old CDs and DVDs that were previous backups. My work does something similar for tape drive data.

DVDs use Reed–Solomon coding, so they effectively store a hash and recovery data for you. When a sector is irrecoverable, reading that sector fails.

For this purpose, I think it would be nice to access the raw data, to see any errors that would be otherwise masked. As someone in the comments suggested, one might compare number of corrected errors in 1, 2, 5 years and compare to the number of redundant bits stored to estimate the expected longevity of the medium

dvdisaster might already be able to do this analysis.

If you are willing to sacrifice some storage space on the disk, then dvdisaster (https://en.wikipedia.org/wiki/Dvdisaster) can add extra ECC data to the disk that will allow recovery even if some percentage of the disk errors out upon read later.

Granted, if one no longer has the mechanical drive, or if the disk errors out beyond the threshold where the extra ECC can correct the errors, the data's still lost. But it (dvdisaster) does provide some protection from the "bit-rot" case where the disk slowly degrades.

Par2 is also very good for resilient storage. It uses parity files that can reconstruct bitrotted files. https://en.wikipedia.org/wiki/Parchive

Also, the storing it outside isn't a very good test of how long it will last inside.

Also also, M-Disc is like Imax, a theater could have that label because it projects 70mm film into a dome or because it's a regular movie theater with a lower resolution than your phone screen that licenses the rights to the name. There are M-Disc DVDs that use a special archival technology that requires compatible drives, but the M-Disc Blu-Ray discs are made with regular Blu-Ray manufacturing technology. With both Imax and M-Disc, they require a minimum quality level to license the trademark, but exceeding that quality level is far from exclusive to that trademark.

My understanding is that the thing that makes M-Disc DVDs special is that they don't use organic dyes in the recording. Blu-ray discs, with the exception of the weird LTH ones, by default don't use organic dyes. Consequently, the main magic of DVD M-Disc is just the default with BDR.

For a long time the vast majority of DVD-R disks have been light-to-dark (ie, the laser writing to a spot makes that spot darker, not lighter.) Dark-to-light disks were rare, the cheapest, and fell out of production pretty fast.

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