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126 lines
4.6 KiB
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QEMU VM templating
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==================
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This document explains how to use VM templating in QEMU.
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For now, the focus is on VM memory aspects, and not about how to save and
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restore other VM state (i.e., migrate-to-file with ``x-ignore-shared``).
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Overview
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--------
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With VM templating, a single template VM serves as the starting point for
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new VMs. This allows for fast and efficient replication of VMs, resulting
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in fast startup times and reduced memory consumption.
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Conceptually, the VM state is frozen, to then be used as a basis for new
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VMs. The Copy-On-Write mechanism in the operating systems makes sure that
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new VMs are able to read template VM memory; however, any modifications
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stay private and don't modify the original template VM or any other
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created VM.
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!!! Security Alert !!!
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----------------------
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When effectively cloning VMs by VM templating, hardware identifiers
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(such as UUIDs and NIC MAC addresses), and similar data in the guest OS
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(such as machine IDs, SSH keys, certificates) that are supposed to be
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*unique* are no longer unique, which can be a security concern.
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Please be aware of these implications and how to mitigate them for your
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use case, which might involve vmgenid, hot(un)plug of NIC, etc..
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Memory configuration
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--------------------
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In order to create the template VM, we have to make sure that VM memory
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ends up in a file, from where it can be reused for the new VMs:
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Supply VM RAM via memory-backend-file, with ``share=on`` (modifications go
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to the file) and ``readonly=off`` (open the file writable). Note that
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``readonly=off`` is implicit.
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In the following command-line example, a 2GB VM is created, whereby VM RAM
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is to be stored in the ``template`` file.
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.. parsed-literal::
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|qemu_system| [...] -m 2g \\
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-object memory-backend-file,id=pc.ram,mem-path=template,size=2g,share=on,... \\
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-machine q35,memory-backend=pc.ram
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If multiple memory backends are used (vNUMA, DIMMs), configure all
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memory backends accordingly.
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Once the VM is in the desired state, stop the VM and save other VM state,
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leaving the current state of VM RAM reside in the file.
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In order to have a new VM be based on a template VM, we have to
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configure VM RAM to be based on a template VM RAM file; however, the VM
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should not be able to modify file content.
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Supply VM RAM via memory-backend-file, with ``share=off`` (modifications
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stay private), ``readonly=on`` (open the file readonly) and ``rom=off``
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(don't make the memory readonly for the VM). Note that ``share=off`` is
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implicit and that other VM state has to be restored separately.
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In the following command-line example, a 2GB VM is created based on the
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existing 2GB file ``template``.
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.. parsed-literal::
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|qemu_system| [...] -m 2g \\
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-object memory-backend-file,id=pc.ram,mem-path=template,size=2g,readonly=on,rom=off,... \\
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-machine q35,memory-backend=pc.ram
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If multiple memory backends are used (vNUMA, DIMMs), configure all
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memory backends accordingly.
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Note that ``-mem-path`` cannot be used for VM templating when creating the
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template VM or when starting new VMs based on a template VM.
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Incompatible features
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---------------------
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Some features are incompatible with VM templating, as the underlying file
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cannot be modified to discard VM RAM, or to actually share memory with
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another process.
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vhost-user and multi-process QEMU
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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vhost-user and multi-process QEMU are incompatible with VM templating.
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These technologies rely on shared memory, however, the template VMs
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don't actually share memory (``share=off``), even though they are
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file-based.
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virtio-balloon
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~~~~~~~~~~~~~~
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virtio-balloon inflation and "free page reporting" cannot discard VM RAM
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and will repeatedly report errors. While virtio-balloon can be used
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for template VMs (e.g., report VM RAM stats), "free page reporting"
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should be disabled and the balloon should not be inflated.
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virtio-mem
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~~~~~~~~~~
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virtio-mem cannot discard VM RAM that is managed by the virtio-mem
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device. virtio-mem will fail early when realizing the device. To use
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VM templating with virtio-mem, either hotplug virtio-mem devices to the
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new VM, or don't supply any memory to the template VM using virtio-mem
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(requested-size=0), not using a template VM file as memory backend for the
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virtio-mem device.
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VM migration
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~~~~~~~~~~~~
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For VM migration, "x-release-ram" similarly relies on discarding of VM
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RAM on the migration source to free up migrated RAM, and will
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repeatedly report errors.
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Postcopy live migration fails discarding VM RAM on the migration
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destination early and refuses to activate postcopy live migration. Note
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that postcopy live migration usually only works on selected filesystems
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(shmem/tmpfs, hugetlbfs) either way.
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