> Hello, all,

> We have recently opened a review on Phabricator for the warm migration code 
> for > bhyve [1]. Please take a look and let us know if it is anything we can 
> improve.

> [1] https://reviews.freebsd.org/D28270

> Thank you,
> Elena

I appreciate that this isn't really related to the current review, and commend 
the work being put into bhyve - it's an invaluable addition to FreeBSD. I'm 
just wondering if any thought has been put into the future possibility for 
transfer of disk data during migration (i.e. the equivalent of "storage 
vmotion")

The current process (from a mile high overview) effectively seems to be the 
following -

* Start guest on host2 pointing at a shared disk, and halt any execution
* use bhyvectl to pause and begin migration on host1
* Start the guest on host2

What would be the feasibility of being able to run a process such as the 
following? Obviously it would likely need to be orchestrated by an external 
tool, but to me it seems the main requirement is really just to be able to 
provide separate control over the pause and migrate steps on host1 -

* send a ZFS snapshot of the running machine to host2
* start the guest in migrate recv mode on host2
* pause the guest on host1
* send a new snapshot
* initiate the migration of memory/device data
* start guest on host2

Are there any major complications here I'm not aware of other than the 
requirement to pause the guest and kick off the state migration as two separate 
calls?

Shared storage is great but becomes complex and expensive if you want high 
performance and reliability (seeing as the storage quickly becomes a major 
single point of failure without enterprise active/active kit). I suspect the 
most common deployment of bhyve is independent hosts with local ZFS pools as 
this is easy/cheap and gives great performance. Most hypervisors only had 
"shared storage" migration for a long time but the big ones now also support 
transferring disk data live. It would be great to be able to do this out of the 
gate.

I did have a poor mans version of this in vm-bhyve, but obviously relied on 
stopping and restarting the guest. I always had problems with the nc commands 
not exiting cleanly though and hanging the process so never made it official.

Regards,
Matt

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