Operating System Design/File Systems/Faults
In engineering in general, fault-tolerance refers to the ability of something to continue to function (though perhaps at reduced levels) after something as gone wrong. More specifically, in filesystem design, it refers to the ability of a filesystem to store data reliably, even in the face of hardware mistakes.
Many things can go wrong in a storage system, especially one with moving parts like hard disk drives. Bad sectors only prevent the use of a few sectors, while a head crash can permanently ruin a disk.
There are several ways of improving fault tolerance in filesystems: