Google has revealed details of a recent incident affecting one of its European data centers, which resulted in an extremely small - but nonetheless permanent - loss of user data.
Logged as 'Google Compute Engine Incident #15056', the company's report details the events that occurred between August 13 and 17:
From 09:19 PDT on Thursday 13 August 2015, to Monday 17 August 2015, some Standard Persistent Disks in the europe-west1-b zone began to return sporadic I/O errors to their connected GCE instances. In total, approximately 5% of the Standard Persistent Disks in the zone experienced at least one I/O read or write failure during the course of the incident. Some management operations on the affected disks also failed, such as disk snapshot creation.
The cause of the problem? As Google explains, the power grid local to the affected data center in Belgium was hit by "four successive lightning strikes... [that] caused a brief loss of power" to storage systems allocated to the 'europe-west1-b' zone. While the data center's emergency power management systems 'quickly' kicked in, Google says that "some recently written data was located on storage systems which were more susceptible to power failure from extended or repeated battery drain."
As a result, Google says that there was "permanent data loss", although this affected only a tiny proportion of the total amount of data hosted in the center - less than 0.000001% of Persistent Disk space in europe-west1-b, according to the company.
Google has accepted full responsibility for the outage and the consequent data loss, and says that its engineers have "conducted a wide-ranging review across all layers of the datacenter technology stack" to identify areas where improvements are needed.