Veera W. Deenadhayalan

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.

Evaluating the impact of Undetected Disk Errors in RAID systems

Despite the reliability of modern disks, recent studies have made it clear that a new class of faults, UndetectedDisk Errors (UDEs) also known as silent data corruption events, become a real challenge as storage capacity scales. While RAID systems have proven effective in protecting data from traditional disk failures, silent data corruption events remain a significant problem unaddressed by RAID. We present a fault model for UDEs, and a hybrid framework for simulating UDEs in large-scale systems. The framework combines a multi-resolution discrete event simulator with numerical solvers. Our implementation enables us to model arbitrary storage systems and workloads and estimate the rate of undetected data corruptions. We present results for several systems and workloads, from gigascale to petascale. These results indicate that corruption from UDEs is a significant problem in the absence of protection schemes and that such schemes dramatically decrease the rate of undetected data corruption.