Amazon will initiate a rolling restart of thousands of Elastic Compute Cloud (EC2) servers over the next four days.

The company didn’t specify the reason for the urgent “maintenance update” but several reports suggest that the blame should be placed on a security bug in the open source Xen hypervisor.

Downtime
"The maintenance will occur sometime during the window provided for each instance. Each instance will experience a clean reboot and will be unavailable while the updates are applied to the underlying host," Amazon told customers in an email.

"We will need to do this maintenance update in the window provided. You will not be able to stop/start or re-launch instances in order to avoid this maintenance update."

The process should take a few minutes before servers return to normal operation.

An unnamed AWS customer told Australian publication ITnews that the update is needed to fix a critical vulnerability in Xen hypervisor which the source says will be revealed to the public on 1 October.

This idea was echoed by Thorsten von Eicken, founder and CTO of the US-based cloud portfolio manager Rightscale: “As usual, AWS is totally tight-lipped about the underlying cause. It seems obvious that the company is patching a security vulnerability, but it will not disclose which one until October 1 — that is, after they have patched all hosts.”

von Eicken advised EC2 users to monitor the ‘events’ page of their AWS console to find out when their servers will be rebooted. He added that all AWS-based applications should be closely monitored during the maintenance window.

The rolling restarts are expected to finish by 30 September.