One of Google's datacenters in Europe, europe-west1-b, suffered permanent data loss after power fluctuations resulted in sporadic I/O errors.

The cause? Four lightning strikes to the power grid disrupted the building's power. Although the datacenter has battery backups and auxiliary generation, Google writes that "some recently written data was located on storage systems which were more susceptible to power failure from extended or repeated battery drain." The result? "[I]n a very few cases, recent writes were unrecoverable, leading to permanent data loss."

The company said that less than 0.000001 percent of storage in its persistent disk system was affected. That doesn't sound like much—it's ten kilobytes for every 1 terabyte—but with the datacenter likely holding many petabytes, it all adds up, and Google could well have lost as much as a few gigabytes of data.

Google was already working on an upgrade program to make its caching system more resilient to power issues; the affected storage was still waiting to be upgraded.