mirror of
https://github.com/qemu/qemu.git
synced 2024-12-16 16:53:28 +08:00
9cd9313fc3
Let's add some details about VM templating, focusing on the VM memory configuration only. There is much more to VM templating (VM state? block devices?), but I leave that as future work. Message-ID: <20230906120503.359863-10-david@redhat.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Signed-off-by: David Hildenbrand <david@redhat.com>
126 lines
4.6 KiB
ReStructuredText
126 lines
4.6 KiB
ReStructuredText
QEMU VM templating
|
|
==================
|
|
|
|
This document explains how to use VM templating in QEMU.
|
|
|
|
For now, the focus is on VM memory aspects, and not about how to save and
|
|
restore other VM state (i.e., migrate-to-file with ``x-ignore-shared``).
|
|
|
|
Overview
|
|
--------
|
|
|
|
With VM templating, a single template VM serves as the starting point for
|
|
new VMs. This allows for fast and efficient replication of VMs, resulting
|
|
in fast startup times and reduced memory consumption.
|
|
|
|
Conceptually, the VM state is frozen, to then be used as a basis for new
|
|
VMs. The Copy-On-Write mechanism in the operating systems makes sure that
|
|
new VMs are able to read template VM memory; however, any modifications
|
|
stay private and don't modify the original template VM or any other
|
|
created VM.
|
|
|
|
!!! Security Alert !!!
|
|
----------------------
|
|
|
|
When effectively cloning VMs by VM templating, hardware identifiers
|
|
(such as UUIDs and NIC MAC addresses), and similar data in the guest OS
|
|
(such as machine IDs, SSH keys, certificates) that are supposed to be
|
|
*unique* are no longer unique, which can be a security concern.
|
|
|
|
Please be aware of these implications and how to mitigate them for your
|
|
use case, which might involve vmgenid, hot(un)plug of NIC, etc..
|
|
|
|
Memory configuration
|
|
--------------------
|
|
|
|
In order to create the template VM, we have to make sure that VM memory
|
|
ends up in a file, from where it can be reused for the new VMs:
|
|
|
|
Supply VM RAM via memory-backend-file, with ``share=on`` (modifications go
|
|
to the file) and ``readonly=off`` (open the file writable). Note that
|
|
``readonly=off`` is implicit.
|
|
|
|
In the following command-line example, a 2GB VM is created, whereby VM RAM
|
|
is to be stored in the ``template`` file.
|
|
|
|
.. parsed-literal::
|
|
|
|
|qemu_system| [...] -m 2g \\
|
|
-object memory-backend-file,id=pc.ram,mem-path=template,size=2g,share=on,... \\
|
|
-machine q35,memory-backend=pc.ram
|
|
|
|
If multiple memory backends are used (vNUMA, DIMMs), configure all
|
|
memory backends accordingly.
|
|
|
|
Once the VM is in the desired state, stop the VM and save other VM state,
|
|
leaving the current state of VM RAM reside in the file.
|
|
|
|
In order to have a new VM be based on a template VM, we have to
|
|
configure VM RAM to be based on a template VM RAM file; however, the VM
|
|
should not be able to modify file content.
|
|
|
|
Supply VM RAM via memory-backend-file, with ``share=off`` (modifications
|
|
stay private), ``readonly=on`` (open the file readonly) and ``rom=off``
|
|
(don't make the memory readonly for the VM). Note that ``share=off`` is
|
|
implicit and that other VM state has to be restored separately.
|
|
|
|
In the following command-line example, a 2GB VM is created based on the
|
|
existing 2GB file ``template``.
|
|
|
|
.. parsed-literal::
|
|
|
|
|qemu_system| [...] -m 2g \\
|
|
-object memory-backend-file,id=pc.ram,mem-path=template,size=2g,readonly=on,rom=off,... \\
|
|
-machine q35,memory-backend=pc.ram
|
|
|
|
If multiple memory backends are used (vNUMA, DIMMs), configure all
|
|
memory backends accordingly.
|
|
|
|
Note that ``-mem-path`` cannot be used for VM templating when creating the
|
|
template VM or when starting new VMs based on a template VM.
|
|
|
|
Incompatible features
|
|
---------------------
|
|
|
|
Some features are incompatible with VM templating, as the underlying file
|
|
cannot be modified to discard VM RAM, or to actually share memory with
|
|
another process.
|
|
|
|
vhost-user and multi-process QEMU
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
vhost-user and multi-process QEMU are incompatible with VM templating.
|
|
These technologies rely on shared memory, however, the template VMs
|
|
don't actually share memory (``share=off``), even though they are
|
|
file-based.
|
|
|
|
virtio-balloon
|
|
~~~~~~~~~~~~~~
|
|
|
|
virtio-balloon inflation and "free page reporting" cannot discard VM RAM
|
|
and will repeatedly report errors. While virtio-balloon can be used
|
|
for template VMs (e.g., report VM RAM stats), "free page reporting"
|
|
should be disabled and the balloon should not be inflated.
|
|
|
|
virtio-mem
|
|
~~~~~~~~~~
|
|
|
|
virtio-mem cannot discard VM RAM that is managed by the virtio-mem
|
|
device. virtio-mem will fail early when realizing the device. To use
|
|
VM templating with virtio-mem, either hotplug virtio-mem devices to the
|
|
new VM, or don't supply any memory to the template VM using virtio-mem
|
|
(requested-size=0), not using a template VM file as memory backend for the
|
|
virtio-mem device.
|
|
|
|
VM migration
|
|
~~~~~~~~~~~~
|
|
|
|
For VM migration, "x-release-ram" similarly relies on discarding of VM
|
|
RAM on the migration source to free up migrated RAM, and will
|
|
repeatedly report errors.
|
|
|
|
Postcopy live migration fails discarding VM RAM on the migration
|
|
destination early and refuses to activate postcopy live migration. Note
|
|
that postcopy live migration usually only works on selected filesystems
|
|
(shmem/tmpfs, hugetlbfs) either way.
|