Bitrot Scrubber - Scrubs your disks (or array) looking for bitrot (silent disk data corruption.)
The size of hard drives has grown steadily over the years, but their bit error rate has remained relatively unchanged. As an example, the Seagate "Barracuda" line has a "Non Recoverable Bit Error per bit Read" rate of 1 bit per 1e14 bits read (Barracuda datasheet). This means that 1 bit may be incorrectly reported every 1e14 bits read from the drive, or 11 TB (bytes) of data. This may seem a lot, but with 4-6+ TB drives becoming commonplace, the problem becomes very obvious.
Some forms of read errors are detectable by the drive firmware. These errors are usually very visible to the system operator and generate ominous error messages in the system log. If a disk array is used, such conditions will normally cause the offending drive to be marked as defective and removed from the array. Our main concern are the "invisible" bit read errors, popularly known as bitrot, which are not detected by the drive firmware and could cause silent data corruption.
The "proper" solution for bitrot protection is to use an advanced file system such as ZFS, which was specifically created to address data integrity issues. Unfortunately, not everybody is willing to move to ZFS and deal with the resulting technical complications or licensing structure.
This program is a simple alternative to detect silent data corruption on disk drives. The idea is to "scrub" your collection of files periodically, so that an eventual corruption might be detected and the file restored from backups. This program will not protect your data from write errors or cache/buffer corruptions in memory (use ECC RAM!). Also, it was not meant to be used as a security audit tool (it is, in fact, useless in that capacity as it purposedly ignores files with metadata changes.)
The principle of operation is simple: Run "bitrot " periodically and the program will read all regular files under that directory recursively, calculating their MD5 hashes. The program then compares the MD5 hash of each file read from disk with a saved version from a previous run. If a file exists with the same filesystem metadata (modification date, size) and the MD5 hash doesn't match, the scrubber assumes data corruption happened and reports the file. The program assumes that modified files (metadata changes) contain valid modifications and it won't try to compare the MD5 hash on those. Instead, the current MD5 file is saved for future runs.