Virtual machine block substitution

One or more computer-readable storage devices storing information to enable a computer to perform a process, the process performed by a virtualization layer executing on the computer, the process comprising:

• executing the virtualization layer on the computer, the virtualization layer managing execution of a virtual machine on the computer, wherein a guest operating system is executing on the virtual machine, the virtualization providing the virtual machine with block-based access to a first virtual machine image, the first virtual machine image comprised of first blocks that the virtualization layer presents as a virtualized disk;
• while the virtual machine is being executed with the first virtual machine image on the computer by the virtualization layer that manages virtual machines on the computer, receiving a second block obtained from a second virtual machine image;
• and
• while the virtual machine continues to be executed, replacing one of the first blocks with the second block.

A method performed by one or more computers, the method comprising:

• applying a patch to a first virtual machine image to generate a second virtual machine image;
• identifying blocks of the second virtual machine image that differ from the first virtual machine image;
• distributing the identified blocks to a plurality of hosts, each host having a hypervisor and one or more virtual machines managed thereby, each virtual machine having a virtual machine image comprised of blocks and presented as a virtual disk by the corresponding hypervisor;
• and
• applying the blocks to the virtual machines as they are running on the hosts, the applying performed by respective hypervisors replacing blocks of the virtual machine images managed by the hypervisors.