Matt Churchyard wrote this message on Mon, Jan 25, 2021 at 10:46 +0000:
> -----Original Message-----
> From: John-Mark Gurney <j...@funkthat.com>
> Sent: 25 January 2021 06:21
> To: Matt Churchyard <matt.churchy...@userve.net>
> Cc: Elena Mihailescu <elenamihailesc...@gmail.com>;
> freebsd-virtualization@freebsd.org
> Subject: Re: Warm Migration feature for bhyve - review on Phabricator
>
> Matt Churchyard wrote this message on Fri, Jan 22, 2021 at 10:09 +0000:
> > > 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?
>
> > There's also hastd which can aid with this...
>
> Thanks for the reply. I've always been wary of the additional complexity of
> HAST and ZFS, as it doesn't seem to have widespread usage or support, and
> things get ugly fast when storage is involved.
Totally agree...
> However, the idea of using HAST on top of zvols to provide network mirrored
> storage for a guest is interesting. It adds a lot of extra complexity, and
> probably performance impact though if it's just for the ability to move a
> guest between systems that may only happen every now and then. I'm also not
> sure it would help (or would at least be even more complex) if I have 4 hosts
> and want to be able to move guests anywhere.
gmirror + ggate is another option as well...
> The main reason for the email was to explore my theory that, by leveraging
> ZFS, any bhyve user could roll their own storage migration ability with a few
> commands as long as the following two (seemingly minor) abilities were present
>
> 1) the ability to suspend a guest without it being done as part of the
> migrate call. (I assume suspend/resume support via bhyvectl is planned
> anyway, if not already in place at this point)
Yeah, I'd hope that there would be ability to insert custom commands
in between the suspend on one host and the resumption on the new host..
But it is good explicitly state this..
> 2) modification of the migrate call to skip suspend if the guest is already
> suspended.
>
> The main thing I'm not sure of is whether the migrate call has any specific
> reliance on doing the suspend itself (e.g. if it needs to do anything before
> the suspend, which will obviously be problematic if suspend & migrate are
> called separately). Or if there's something else I missed that means this is
> not feasible. I'm not really after massive changes to the current review to
> implement disk migration in bhyve itself.
+1
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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