1) What's this
When the migration capability 'bypass-shared-memory'
is set, the shared memory will be bypassed when migration.
It is the key feature to enable several excellent features for
the qemu, such as qemu-local-migration, qemu-live-update,
extremely-fast-save-restore, vm-template, vm-fast-live-clone,
yet-another-post-copy-migration, etc..
The philosophy behind this key feature, including the resulting
advanced key features, is that a part of the memory management
is separated out from the qemu, and let the other toolkits
such as libvirt, kata-containers (https://github.com/kata-containers)
runv(https://github.com/hyperhq/runv/) or some multiple cooperative
qemu commands directly access to it, manage it, provide features on it.
2) Status in real world
The hyperhq(http://hyper.sh http://hypercontainer.io/)
introduced the feature vm-template(vm-fast-live-clone)
to the hyper container for several years, it works perfect.
(see https://github.com/hyperhq/runv/pull/297).
The feature vm-template makes the containers(VMs) can
be started in 130ms and save 80M memory for every
container(VM). So that the hyper containers are fast
and high-density as normal containers.
kata-containers project (https://github.com/kata-containers)
which was launched by hyper, intel and friends and which descended
from runv (and clear-container) should have this feature enabled.
Unfortunately, due to the code confliction between runv&cc,
this feature was temporary disabled and it is being brought
back by hyper and intel team.
3) How to use and bring up advanced features.
In current qemu command line, shared memory has
to be configured via memory-object.
a) feature: qemu-local-migration, qemu-live-update
Set the mem-path on the tmpfs and set share=on for it when
start the vm. example:
-object \
memory-backend-file,id=mem,size=128M,mem-path=/dev/shm/memory,share=on \
-numa node,nodeid=0,cpus=0-7,memdev=mem
when you want to migrate the vm locally (after fixed a security bug
of the qemu-binary, or other reason), you can start a new qemu with
the same command line and -incoming, then you can migrate the
vm from the old qemu to the new qemu with the migration capability
'bypass-shared-memory' set. The migration will migrate the device-state
*ONLY*, the memory is the origin memory backed by tmpfs file.
b) feature: extremely-fast-save-restore
the same above, but the mem-path is on the persistent file system.
c) feature: vm-template, vm-fast-live-clone
the template vm is started as 1), and paused when the guest reaches
the template point(example: the guest app is ready), then the template
vm is saved. (the qemu process of the template can be killed now, because
we need only the memory and the device state files (in tmpfs)).
Then we can launch one or multiple VMs base on the template vm states,
the new VMs are started without the “share=on”, all the new VMs share
the initial memory from the memory file, they save a lot of memory.
all the new VMs start from the template point, the guest app can go to
work quickly.
The new VM booted from template vm can’t become template again,
if you need this unusual chained-template feature, you can write
a cloneable-tmpfs kernel module for it.
The libvirt toolkit can’t manage vm-template currently, in the
hyperhq/runv, we use qemu wrapper script to do it. I hope someone add
“libvrit managed template” feature to libvirt.
d) feature: yet-another-post-copy-migration
It is a possible feature, no toolkit can do it well now.
Using nbd server/client on the memory file is reluctantly Ok but
inconvenient. A special feature for tmpfs might be needed to
fully complete this feature.
No one need yet another post copy migration method,
but it is possible when some crazy man need it.