On 15.07.26 19:00, Peter Xu wrote:
On Wed, Jul 15, 2026 at 04:52:48PM +0100, Daniel P. Berrangé wrote:
On Wed, Jul 15, 2026 at 11:48:40AM -0400, Michael S. Tsirkin wrote:
On Wed, Jul 15, 2026 at 06:01:39PM +0300, Michael Tokarev wrote:
On 14.07.2026 18:42, Vladimir Sementsov-Ogievskiy wrote:
Hi all!
Here is a migration for TAP net backend, including its properties and
open fds.
With this new feature, management software doesn't need to initialize
new TAP and do a switch to it. Nothing should be done around
virtio-net in local migration: it just migrates and continues to use
same TAP device. So we avoid extra logic in management software, extra
allocations in kernel (for new TAP), and corresponding extra delay in
migration downtime.
This is quite a big patch set, - is this really worth the effort to do
all this just for *local* migration? What's the possible use case for
this in real, - am I right this is just about upgrading the host qemu?
And with that in mind, isn't it sufficient to use what we already have
(namely, create new tap, start new qemu instance, and migrate the usual
way), and tolerate some very minor downtime while the networking code
learns the new network topology (isn't it happening almost instantly
anyway, and if not, the management can help by sending gratitious ARP)?
I wonder what's the use for this at yandex?
Thanks,
/mjt
Well just theoretically, imagine a big VM, reserving twice the amount
of memory just to migrate is not nice at all.
Don't we already have the ability to skip memory transfer by setting
the "x-ignore-shared" capability, assuming the VM RAM has a shared
memory backing.
Right, IIUC all similar single-host migrations like this series or CPR (or
anything else...) should always need to enable x-ignore-shared in the first
place. That's almost always the starting point of optimizing local
migrations.. no matter how the memory will be shared (by the same pool of
page cache, or persisted over kexec, etc.).
Yes, sharing RAM between source and target + enabling x-ignore-shared is a
first thing to do.
This series optimizes TAP recreating. Not only skip recreating but also allow to
exclude cloud-networking component form live-update entirely, making the process
simpler (less components involved), and as I already said, reducing
corresponding
downtime.
--
Best regards,
Vladimir
