Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots

[Peter Krempa]

On Thu, Nov 26, 2020 at 18:17:28 +0300, Andrey Gruzdev via wrote:
> This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's
> implemented in his series '[PATCH v0 0/4] migration: add background snapshot'.
> 
> Currently the only way to make (external) live VM snapshot is using existing
> dirty page logging migration mechanism. The main problem is that it tends to
> produce a lot of page duplicates while running VM goes on updating already
> saved pages. That leads to the fact that vmstate image size is commonly 
> several
> times bigger then non-zero part of virtual machine's RSS. Time required to
> converge RAM migration and the size of snapshot image severely depend on the
> guest memory write rate, sometimes resulting in unacceptably long snapshot
> creation time and huge image size.
> 
> This series propose a way to solve the aforementioned problems. This is done
> by using different RAM migration mechanism based on UFFD write protection
> management introduced in v5.7 kernel. The migration strategy is to 'freeze'
> guest RAM content using write-protection and iteratively release protection
> for memory ranges that have already been saved to the migration stream.
> At the same time we read in pending UFFD write fault events and save those
> pages out-of-order with higher priority.

This sounds amazing! Based on your description I assume that the final
memory image contains state image from the beginning of the migration.

This would make it desirable for the 'migrate' qmp command to be used as
part of a 'transaction' qmp command so that we can do an instant disk
and memory snapshot without any extraneous pausing of the VM.

I'll have a look at using this mechanism in libvirt natively.