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26670e4778
Add a section about restoring VM with new Net FDs explicitly passed to ch-remote via 'net_fds' parameter Signed-off-by: Purna Pavan Chandra <paekkaladevi@linux.microsoft.com>
114 lines
3.7 KiB
Markdown
114 lines
3.7 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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## Restore a VM with new Net FDs
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For a VM created with FDs explicitly passed to NetConfig, a set of valid FDs
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need to be provided along with the VM restore command in the following syntax:
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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 net_fds=[net1@[23,24],net2@[25,26]]
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```
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In the example above, the net device with id `net1` will be backed by FDs '23'
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and '24', and the net device with id `net2` will be backed by FDs '25' and '26'
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from the restored VM.
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## Limitations
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VFIO devices and Intel SGX are out of scope.
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