Undetected disk errors in RAID arrays

Though remarkably reliable, disk drives do fail occasionally. Most failures can be detected immediately; moreover, such failures can be modeled and addressed using technologies such as RAID (Redundant Arrays of Independent Disks). Unfortunately, disk drives can experience errors that are undetected by the drive—which we refer to as undetected disk errors (UDEs). These errors can cause silent data corruption that may go completely undetected (until a system or application malfunction) or may be detected by software in the storage I/O stack. Continual increases in disk densities or in storage array sizes and more significantly the introduction of desktop-class drives in enterprise storage systems are increasing the likelihood of UDEs in a given system. Therefore, the incorporation of UDE detection (and correction) into storage systems is necessary to prevent increasing numbers of data corruption and data loss events. In this paper, we discuss the causes of UDEs and their effects on data integrity. We describe some of the basic techniques that have been applied to address this problem at various software layers in the I/O stack and describe a family of solutions that can be integrated into the RAID subsystem.