Amazon Web Services is planning an "urgent patch" that will start on Friday September 26 and carry through until September 30.
The patch will cause a maintenance reboot of EC2 instances and was first publicised by a blog post by RightScale CTO Thorsten Von Eicken.
Amazon said a bug was that was discovered in the Xen virtualisation platform may be the cause. It is not known whether it is a security bug.
Writing on the RightScale blog, Eicken said: "What makes this event different than similar cases in the past, most notably the December 2011 instance reboot, are two characteristics: 1. A substantial number of instances will be rebooted (see below for a list of instance types that are not affected). AWS has said that not all instances of the impacted instance types will be rebooted. 2. If you relaunch an instance before the maintenance, you are not guaranteed to get an already-patched host."
T1, T2, M2, R3, and HS1 instance types will not be affected.
In an email to customers, Amazon said: "One or more of your Amazon EC2 instances are scheduled to be rebooted for required host maintenance.
"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."
Eicken said: "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.
"We’re curious whether this issue affects other cloud providers as well and how they will react. It will be interesting to see whether some cloud providers’ live migration capabilities allow them to handle the event without visible customer impact."
The reboots will start on September 26, 2014, at 2:00 UTC/GMT (September 25, 2014, at 7:00 PM PDT) and end on September 30, 2014, at 23:59 UTC/GMT (September 30, 2014, at 4:59 PM PDT).