Re: [Xen-devel] reliable live migration of large and busy guests

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Subject: Re: [Xen-devel] reliable live migration of large and busy guests
> 
> On 06/11/12 23:41, Dan Magenheimer wrote:
> >> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> >> Sent: Tuesday, November 06, 2012 4:19 PM
> >> To: xen-devel@xxxxxxxxxxxxx
> >> Subject: Re: [Xen-devel] reliable live migration of large and busy guests
> >>
> >> As potential food for thought:
> >>
> >> Is there wisdom in having a new kind of live migrate which, when pausing
> >> the VM on the source host, resumes the VM on the destination host.  Xen
> >> would have to track not-yet-sent pages and pause the guest on pagefault,
> >> and request the required page as a matter of priority.
> >>
> >> The advantages of this approach would be that a timing sensitive
> >> workloads would be paused for far less time.  Even if it was frequently
> >> being paused for pagefaults, the time to get a single page over the LAN
> >> would be far quicker than the entire dirty set, at which point on
> >> resume, the interrupt paths would fire again; The timing paths would
> >> quickly become fully populated.  Further to that, a busy workload in the
> >> guest dirtying a page which has already been sent will not result in any
> >> further network traffic.
> > Something like this?
> >
> > http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.184.2368
> 
> Oh wow - something quite like that.  Thankyou very much.  I will read
> the paper in full when I get a free moment, but the abstract looks very
> interesting.

Hi Andrew --

FYI, selfballooning is now built into the Linux kernel (since about
summer of 2011, so may not be in many distros yet).  It is currently
tied to tmem (transcendent memory), which is not turned on by
default but if you start developing something like post-copy migration,
let me know.  AFAIK, there is no way to do selfballooning in
Windows (not even in userspace I think, since IIRC, unlike Linux
sysfs, there is no way to adjust the balloon size outside the
kernel... but I know nothing about Windows ;-)
 
> From an idealistic point of view, it might be quite nice to have several
> live migrate mechanisms, so the user can choose whether they value
> minimum downtime, minimum network utilisation, or maximum safety.

Agreed.  IIRC, when post-copy was suggested for Xen years ago,
Ian Pratt was against it, though I don't recall why, so Michael
Hines work was never pursued (outside of academia).  Probably
worth asking IanP before investing too much time into it.

Dan

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