Comment by littlestymaar
1 year ago
You assume that the hardware would be unneeded, but that's a very strong assumption.
It would be very bad for any cloud provider to leave hosts with only one VM running on it, and you can be pretty sure only very small minority of their park that end up in that situation where shutting down a single VM would lead to a shut-down of the entire host, because it means that the host was vastly under-used in the first place.
As far as I know, most cloud hosts don't actually support automatically moving live VMs, so I think it's fairly common for a host to be left running a single VM.
At least in AWS, they never supported this, and in fact may require you to reboot an instance occasionally in order for it to be moved to a new hardware host (typically when they are upgrading their hardware).
But why are you talking about moving VMs?! Looks like you're adding tons of far-fetched speculations at every step of your reasoning.
The way you easily deal with this issue is very simple and does not require moving VMs: you just allocate newly spawned VMs to existing hosts with available room! When you do so (and they obviously all do!) you end up with little unused hardware…
Say you have 3 hosts, each with a capacity of 10 VMs. At some point you have 28 running VMs - 10 on host1, 10 on host2, 8 on host3. Someone then closes down 2 of the VMs on host1, and 7 of the VMs on host3.
Now you have 19 VMs running, but need to keep all 3 hosts powered. If you don't have live VM moving, you are now forced to keep Host3 running only because 1 VM is running on it, even if that VM is idle. So, this one idle VM is responsible for all the energy consumption of host3, and will continue to be so until at least 3 more VMs get started (since you have room for 2 more VMs on host1).
If you did have live VM migration, or if the idle VM were powered down instead of running idle, you could close host3 completely, moving the VM to host 1, and only re-open host3 if needed for new VMs.
This is equivalent to the problem of memory fragmentation. Even though overall usage is low, if host usage is highly fragmented and you aren't allowed to move used memory around (compacting), you can end up consuming far more than actually needed.
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