Google has admitted that some of its users have lost data stored in its cloud following a lightning strike on one of its European data centers last month.

Google’s facility in St. Ghislain, Belgium, was hit by no less than four lightning strikes between August 13 and August 17, causing errors on its Compute Engine’s persistent disks. The errors were also the cause of a minor service outage experienced by some of its European customers.

According to Google, the lightning strikes resulted in a loss of power to some of its storage systems, which led to the persistent disk errors.

“At 09:19 PDT on Thursday 13 August 2015, four successive lightning strikes on the electrical systems of a European datacenter caused a brief loss of power to storage systems which host disk capacity for GCE instances in the europe-west1-b zone,” Google confessed in an incident report earlier this week.

“Although automatic auxiliary systems restored power quickly, and the storage systems are designed with battery backup, some recently written data was located on storage systems which were more susceptible to power failure from extended or repeated battery drain.”

Google said that about five percent of its persistent disks in the europe-west1-b zone experienced at least one I/O read or write failure during the five day period when the problems occurred. As a result, a very small fraction of the persistent-disk space permanently lost some data – around 0.000001 percent, Google’s engineers said.

Despite not having any control over the weather, Google did assume full responsibility for the outage and data loss.

“This outage is wholly Google’s responsibility,” the company said.

To try and prevent this from happening again, Google’s infrastructure team is in the process of replacing its storage systems with newer hardware that’s less susceptible to power failures.

“Since the incident began, Google engineers have conducted a wide-ranging review across all layers of the data centre technology stack, from electrical distribution systems through computing hardware to the software controlling the GCE persistent disk layer,” the company said.

Google also took the opportunity to remind its customers that it operates multiple cloud regions across the globe with multiple isolated zones. The point of this is that users can setup resilient infrastructure that’s capable of failing over from one zone to another in the case of any problems like we saw this month.

Google Compute Engine operates in three regions: Central US in Council Bluffs, Iowa, Western Europe in St. Ghislain, and East Asia in Changhua County, Taiwan. The Central US region operates four isolated zones, while East Asia and Western Europe possess three each.

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