warehousing and analytics, faces silent corruption every 15 minutes. As another example, a real-life study performed by NetApp on more than 1.5 million HDDs over 41 months found more than 400,000 silent data corruptions, out of which more than 30,000 were not detected by the hardware RAID controller. Another study, performed by CERN over six months and involving about 97 petabytes of data, found that about 128 megabytes of data became permanently corrupted. Silent data corruption may result in cascading failures, in which the system may run for a period of time with undetected initial error causing increasingly more problems until it is ultimately detected. For example, a failure affecting file system metadata can result in multiple files being partially damaged or made completely inaccessible as the file system is used in its corrupted state. Countermeasures When data corruption behaves as a Poisson process, where each bit of data has an independently low probability of being changed, data corruption can generally be detected by the use of checksums, and can often be corrected by the use of error correcting codes (ECC). If an uncorrectable data corruption is detected, procedures such as automatic retransmission or restoration from backups can be applied. Certain levels of RAID disk arrays have the ability to store and evaluate parity bits for data across a set of hard disks and can reconstruct corrupted data upon the failure of a single or multiple disks, depending on the level of RAID implemented. Some CPU architectures employ various transparent checks to detect and mitigate data corruption in CPU caches, CPU buffers and instruction pipelines; an example is Intel Instruction Replay technology, which is available on Intel Itanium processors. Many errors are detected and corrected by the hard disk drives using the ECC codes which are stored on disk for each sector. If the disk drive detects multiple read errors on a sector it may make a copy of the failing sector on another part of the disk, by remapping the failed sector of the disk to a spare sector without the involvement of the operating system (though this may be delayed until the next write to the sector). This "silent correction" can be monitored using S.M.A.R.T. and tools available for most operating systems to automatically check the disk drive for impending failures by watching for deteriorating SMART parameters. Some file systems, such as Btrfs, HAMMER, ReFS, and ZFS, use