RAID protects you against one and only one thing: a disk failure. It does
not protect you against any of the following things:
- Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6), including possible UREs.
- Failure of the RAID controller itself (if applicable), the computer running the RAID, or the environment containing the servers (e.g. a flood, fire, or theft).
- Data corruption from filesystem bugs, cosmic rays, or minor hardware or firmware failures, which can and do happen all the time - you usually just don’t notice and software works around it.
- Malicious or accidental deletion or modification of files, including by viruses, bad application writes, or administrative mistakes (e.g. rm-ing the wrong file or mkfs on an existing filesystem).
The adage is simple: “RAID replicates
everything, instantly, even the stuff you don’t want it to.”
