We are currently hosting about 10 of our own various websites on a virtualised host. Each website is hosted by its own virtual machine with its own apache. Some of the sites connect to a shared database (MySQL) some use a different locally hosted database (again MySQL).
We're starting to run out of resources to allocate to virtual machines as there are now quite a few separate VMs. There are clearly some websites that are busier than others.
I've been considering combining a few websites onto a single host to reduce the total number of VMs to say 4.
The upshot of doing this would be to reduce the overhead required for each website but of course with that comes reduced stability; if Apache/MySQL/PHP fails for whatever reason (or needs restarting) a cluster of websites goes down rather than just one.
My question is: at what point does a website deserve its own dedicated VM?