Tuesday, August 16, 2022

Optical Media Durability Update

Four years ago I posted Optical Media Durability and discovered:
Surprisingly, I'm getting good data from CD-Rs more than 14 years old, and from DVD-Rs nearly 12 years old. Your mileage may vary.
Three years ago I repeated the mind-numbing process of feeding 45 disks through the reader and verifying their checksums. Two years ago I did it again, and then again a year ago.

It is time again for this annual chore, and yet again this year every single MD5 was successfully verified. Below the fold, the details.

The fields in the table are as follows:
  • Month: The date marked on the media in Sharpie, and verified via the on-disk metadata.
  • Media: The type of media.
  • Good: The number of media with this type and date for which all MD5 checksums were correctly verified.
  • Bad: The number of media with this type and date for which any file failed MD5 verification.
  • Vendor: the vendor name on the media
The drives I use from ASUS and LG report read errors from the CDs but verify the MD5s correctly. I didn't notice them reporting any read errors from the DVDs. An off-brand drive fails to read the CDs, but read one of the older DVDs with no read errorrs.

Surprisingly, with no special storage precautions, generic low-cost media, and consumer drives, I'm getting good data from CD-Rs more than 18 years old, and from DVD-Rs nearly 16 years old. Your mileage may vary. Tune in again next year for another episode.

During the year I found a NetBSD1.2 CD dating from October 1996. It has checksums generated by cksum(1), all of which verified correctly despite a few read errors. So that CD is delivering good data after more than a quarter-century.


Hiccup said...

I'm curious, are the checksums of the whole disc image or just part of it (e.g. ISO filesystem/file(s))?

David. said...

There is an MD5 for each file on the disk. For most of the DVDs there are four 1GB files, so the disk is nearly full. The CDs typically have more files, necessarily smaller, but are also nearly full.

Tardigrade said...

Can this be extrapolated to "pressed" CDs and DVDs?

I'm thinking another good test for CDs and DVDs without checksums would be to calculate the checksums from two copies and then compare them (or bitwise compare them). Does anyone use two or more optical disks containing identical data as an error-resistant RAID?