What Is Bit Rot?
Bit rot is a form of data degradation - what you have written before is not exactly the same as what is read now. In binary terms it means a value originally written down as 1 is read back as 0.
The following illustrate the effects of bit rot on raw data type unsigned char (used to represent whole numbers from 0 to 255).
| Original Bits | Original Value | Corrupted Bit(s) Representation | Corrupted Value |
|---|---|---|---|
|
1111 1111
|
255 | 1111 1110 | 254 |
| 0111 1111 | 127 | ||
| 1111 0111 | 247 | ||
| 0000 0000 | 0 | 0000 0000 | 1 |
| 0010 0000 | 32 | ||
| 1000 0000 | 128 |
Note the change in the corrupted value depends on position of the corrupted bit. This is the same table from The Basics Of Bits (Part II). This table highlights the significance of the jump in value from the LSB to the MSB.
| Bit Position | To The Power Of | Binary Value |
|---|---|---|
| 1 | o or 10 | o or 1 |
| 2 | o or 21 | 0 or 2 |
| 3 | o or 22 | 0 or 4 |
| 4 | o or 23 | 0 or 8 |
| 5 | o or 24 | 0 or 16 |
| 6 | o or 25 | 0 or 32 |
| 7 | o or 26 | 0 or 64 |
| 8 (MSB if unsigned) | o or 27 | 0 or 128 |
| 8 (MSB if signed) | 0 or 10 | 1 or -1 |
The difference of bit rot on char data type can be small, the difference can range from insignificant change of 1 to half the value of the data type, 128. Both are not big numbers by most measures. Where it can make a difference is in passwords used in some instances. e.g. if you have bit rot happened on the space where your password is saved, in the worst case you can no longer login to your OS or access the application software.
This problem can scale up with more complex building blocks. On other data types (e.g. 4 byte integers), a bit flip can alter the final value by 231 (That’s a value of 2,147,483,648)! With more complicated encoding that works on multiple bytes in a single block (e.g. a FLAC frame), a single corrupted bit means the entire block has to be discarded.
In general, as the complexity of the bits representation increases, the worse the effects of bit rot can be. However, having said that, some encodings have various mechanisms like built in error detection, fallback, backup and correction.
So again let me stress this is nothing to be alarmed about, much! The odds of any of the above happening is really small - your odds of winning division 1 lottery is likely to be higher.
The problem of bit rot only comes when multiple bit rots over time corrupted the data file (or software) enough to cause problems (e.g. random computer crashes, corrupted videos, corrupted audio, etc). Usually by this time it is too late to fix or salvage anything.
Regular backups is no guarantee this data corruption. You’d have to ensure the integrity of the files being backed up as well.
In the next section we’d demonstrate the effects of a single bit rot.
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4 comments
Comment from: Frank Collins Visitor
Comment from: Agent Kith Member
It does a four-weekly scrub which detects and corrects bit rot on the whole file system. While bit rot isn’t a problem for me, it never will be.+1. FreeNAS for the win.
Comment from: Agent Kith Member
Maybe it’s Cosmic Rays: https://science.slashdot.org/story/17/02/19/2330251/serious-computer-glitches-can-be-caused-by-cosmic-rays
A "single-event upset" was also blamed for an electronic voting error in Schaerbeekm, Belgium, back in 2003. A bit flip in the electronic voting machine added 4,096 extra votes to one candidate. The issue was noticed only because the machine gave the candidate more votes than were possible. "This is a really big problem, but it is mostly invisible to the public," said Bharat Bhuva. Bhuva is a member of Vanderbilt University’s Radiation Effects Research Group, established in 1987 to study the effects of radiation on electronic systems.
Comment from: Agent Kith Member
Here is an interesting YouTube video about bit flipping.
I don’t believe bit rot is a problem and the Red Book standard copes with missing bits, to a point.
I store all of my music (FLAC, level 8 compression) on a NAS, using FreeNAS. It does a four-weekly scrub which detects and corrects bit rot on the whole file system. While bit rot isn’t a problem for me, it never will be.