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In order to allow for human readable output for the VM configuration, we pull it out of the snapshot, which becomes effectively the list of states from the VM. The configuration is stored through a dedicated file in JSON format (not including any binary output). Having the ability to read and modify the VM configuration manually between the snapshot and restore phases makes debugging easier, as well as empowers users for extending the use cases relying on the snapshot/restore feature. Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
99 lines
3.1 KiB
Markdown
99 lines
3.1 KiB
Markdown
# Snapshot and Restore
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The goal for the snapshot/restore feature is to provide the user with the
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ability to take a snapshot of a previously paused virtual machine. This
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snapshot can be used as the base for creating new identical virtual machines,
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without the need to boot them from scratch. The restore codepath takes the
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snapshot and creates the exact same virtual machine, restoring the previously
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saved states. The new virtual machine is restored in a paused state, as it was
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before the snapshot was performed.
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## Snapshot a Cloud Hypervisor VM
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First thing, we must run a Cloud Hypervisor VM:
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```bash
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--cpus boot=4 \
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--memory size=4G \
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--kernel vmlinux \
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--cmdline "root=/dev/vda1 console=hvc0 rw" \
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--disk path=focal-server-cloudimg-amd64.raw
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```
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At any point in time when the VM is running, one might choose to pause it:
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```bash
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock pause
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```
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Once paused, the VM can be safely snapshot into the specified directory and
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using the following command:
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```bash
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock snapshot file:///home/foo/snapshot
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```
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Given the directory was present on the system, the snapshot will succeed and
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it should contain the following files:
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```bash
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ll /home/foo/snapshot/
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total 4194536
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drwxrwxr-x 2 foo bar 4096 Jul 22 11:50 ./
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drwxr-xr-x 47 foo bar 4096 Jul 22 11:47 ../
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-rw------- 1 foo bar 1084 Jul 22 11:19 config.json
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-rw------- 1 foo bar 4294967296 Jul 22 11:19 memory-ranges
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-rw------- 1 foo bar 217853 Jul 22 11:19 state.json
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```
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`config.json` contains the virtual machine configuration. It is used to create
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a similar virtual machine with the correct amount of CPUs, RAM, and other
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expected devices. It is stored in a human readable format so that it could be
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modified between the snapshot and restore phases to achieve some very special
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use cases. But for most cases, manually modifying the configuration should not
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be needed.
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`memory-ranges` stores the content of the guest RAM.
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`state.json` contains the virtual machine state. It is used to restore each
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component in the state it was left before the snapshot occurred.
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## Restore a Cloud Hypervisor VM
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Given that one has access to an existing snapshot in `/home/foo/snapshot`,
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it is possible to create a new VM based on this snapshot with the following
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command:
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```bash
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./cloud-hypervisor \
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--api-socket /tmp/cloud-hypervisor.sock \
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--restore source_url=file:///home/foo/snapshot
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```
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Or using two different commands from two terminals:
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```bash
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# First terminal
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./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock
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# Second terminal
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock restore source_url=file:///home/foo/snapshot
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```
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Remember the VM is restored in a `paused` state, which was the VM's state when
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it was snapshot. For this reason, one must explicitly `resume` the VM before to
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start using it.
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```bash
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./ch-remote --api-socket=/tmp/cloud-hypervisor.sock resume
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```
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At this point, the VM is fully restored and is identical to the VM which was
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snapshot earlier.
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## Limitations
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VFIO devices and Intel SGX are out of scope.
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