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Amazon (s AMZN) is planning to reboot an undetermined number of customer EC2 instances in the coming week or so, something that caused a stir among Amazon watchers today.

Randy Bias, CTO and founder of Cloudscaling, an authority on cloud services, first reported what appeared to be the scheduled reboot of “hundreds to thousands” of EC2 instances early Wednesday afternoon and followed his initial Twitter post and blog with live updates.

A check of the Amazon dashboard at 3:45 p.m. EST showed operations running normally except in the Northern Virginia data center which reported “problems with ELB scaling and provisioning” for the eastern U.S. region. ELB is elastic load balancing which distributes a user’s application load across multiple EC2 instances.

An Amazon spokeswoman said via email that EC2 services are running normally and that the reboots are scheduled and customers are informed of them in advance. The company upgrades the EC2 frequently, often in a way that is invisible to customers, but some updates require that instances be restarted, she said. “That’s what you’re seeing discussed today as we’ve sent customers the schedule.”

Customers can also restart their instances earlier than the scheduled time if they prefer.

“This rollout schedule matches pretty closely with the maintenance schedule you might see from traditional hosting providers or internal IT groups when they roll out software patches or updates. We are careful not to perform updates to multiple Availability Zones in the same Region on the same day so that customers won’t have instances in different Availability Zones update on the same day. We’re also giving notice a few days in advance for this maintenance window,” the spokeswoman added.

Bias published a redacted letter Amazon sent to a customer on December 6 with reboots slated to begin on December 15.

Amazon did not specify what the patches addressed, although some Amazon watchers think they may have to do with Xen hypervisor issues.

Photo courtesy of Flickr user liber.