[PATCH v2 9/9] docs: Start documenting VM templating

[David Hildenbrand <david@redhat.com>]

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.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 docs/vm-templating.txt | 109 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 docs/vm-templating.txt

diff --git a/docs/vm-templating.txt b/docs/vm-templating.txt
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+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.
+
+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.
+
+ qemu [...] -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'.
+
+ qemu [...] -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.
-- 
2.41.0