Some Facebook users were unable to access their digital photographs posted on the social networking site last weekend.
According to a Facebook log, several drives failed at once during a routine upgrade on Friday night, resulting in around 15 percent of members being unable to view their images.
"You may have noticed in the past day that some photos aren't appearing or are displaying a 'question mark' graphic when you go to view them. We're trying to fully understand what happened, since simultaneous hardware failures like this are rare," Evan Priestley, a Facebook engineer, said.
Facebook said its users' photos are safe because it stores multiple copies of the data for disaster recovery and business continuity purposes, and it is working to make the photos affected by the system failure available again as soon as possible.
"We've already repaired about one-third of affected photos and expect to complete repairs on another third tonight," Priestley said. "However, even though your photos are safe, we can't serve photos off the affected storage volumes until they're repaired."
The repair process involves copying large amounts of data to new drives, Priestley said. "This is why some photos aren't showing up right now."
Facebook is restoring photos as it repairs the hard drives, so some of those drives will likely be working again today. The social network site should be back to normal by early next week, it said.
"New photo uploads will continue to work properly during the repairs, because we write them to different storage volumes. Thanks for bearing with us while we return things to normal," Priestley said.
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