An analysis of OpenStack Shelve/unshelve/stop

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The stop virtual machine simply stops the virtual machine and does not release the virtual machine at hypervisor. The virtual machine still occupies a temporary storage resource.
The system also retains cpu/memory resources for the virtual machine to ensure that the boot succeeds.

When we need to completely release cpu/memory/temporarily storage resources, we introduce shelve.
The shelve principle is to release the virtual machine from the hypervisor. The cpu/memory of the virtual machine is no longer reserved.
For temporary storage, we place the disk data on the glance with a snapshot.
We don't need to be so complicated with cinder back-end storage.

The shelve consists of three operations:

Shelve: Stops the virtual machine and sets it to the shelved state. Take a snapshot of the temporary storage and store it on the glance.
Wait for shelve-offload operation or shelve cycle to arrive.
Shelve-offload: Shelved virtual machine is completely released from Hypervisor. The virtual machine state transitions to shelve-offload.
Unshelve: Once again, select the node to start the virtual machine in the Shelve-offload state. (The image on glance will be deleted later)

A recurring task:

Persist the virtual machine in the shelved state, and release the virtual machine from the hypervisor when the shelve cycle arrives.

Disadvantages of existence:

1. Unshelve may fail because of insufficient resources.
2. Boot speed is slower than stop virtual machine boot. Especially temporary storage (need to download the image).
3. Only take a snapshot of the system disk, so other disk data is lost (temporarily stored).
4. Because of the snapshot, the temporarily stored differential volume and the master volume are merged, so the temporary storage consumed after the unshelve is increased. (equivalent to multiple copies of a master volume)

An analysis of OpenStack Shelve/unshelve/stop