Amazon has started warning customers of its EC2 cloud computing platform that it's going to start rebooting their instances en masse over the next few days.

Not all EC2 instance types will need rebooting; the fast storage (HS1) and large memory (R3) virtual machines are unaffected. But the reboots will nonetheless be widespread, with all regions and all availability zones affected. Rebooting is due to start this evening and finish next Tuesday.

Amazon hasn't explained why the bulk reboot is needed, but a source speaking to ITNews has said that it's due to an embargoed flaw in the Xen hypervisor. That bug, XSA-108, is due to be publicly disclosed on October 1, by which time Amazon's reboot should be complete.