12.12

What is scrubbing, in the context of RAID systems, and why is scrubbing important?

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Even if all writes are completed properly, there is a small chance of a sector in a disk becoming unreadable at some point, even though it was successfully written earlier. Reasons for loss of data on individual sectors could range from manufacturing defects to data corruption on a track when an adjacent track is written repeatedly. Such loss of data that were successfully written earlier is sometimes referred to as a latent failure, or as bit rot 💩. When such a failure happens, if it is detected early the data can be recovered from the remaining disks in the RAID organization. However, if such a failure remains undetected, a single disk failure could lead to data loss if a sector in one of the other disks has a latent failure (assuming RAID level 5 is used).

To minimize the chance of such data loss, good RAID controllers perform scrubbing; that is, during periods when disks are idle, every sector of every disk is read, and if any sector is found to be unreadable, the data can be recovered from the remaining disks in the RAID organization, and the sector is written back. (If the physical sector is damaged, the disk controller would remap the logical sector address to a different physical sector on disk).