Scrubbing

Given all of the discussion in this chapter, you might be wondering: when do these checksums actually get checked? Of course, some amount of checking occurs when data is accessed by applications, but most data is rarely accessed, and thus would remain unchecked. Unchecked data is problematic for a reliable storage system, as bit rot could eventually affect all copies of a particular piece of data.

To remedy this problem, many systems utilize disk scrubbing“Parity Lost and Parity Regained” by Andrew Krioukov, Lakshmi N. Bairavasundaram, Garth R. Goodson, Kiran Srinivasan, Randy Thelen, Andrea C. Arpaci-Dusseau, Remzi H. Arpaci-Dusseau. FAST ’08, San Jose, CA, February 2008. This work explores how different checksum schemes work (or don’t work) in protecting data. We reveal a number of interesting flaws in current protection strategies. of various forms. By periodically reading through every block of the system, and checking whether checksums are still valid, the disk system can reduce the chances that all copies of a certain data item become corrupted. Typical systems schedule scans on a nightly or weekly basis.