[openstack-dev] [nova] Enabling VM post-copy live migration

Luis Tomas luis at cs.umu.se
Thu Mar 12 12:26:12 UTC 2015

On 03/12/2015 12:34 PM, John Garbutt wrote:
> On 12 March 2015 at 08:41, Luis Tomas <luis at cs.umu.se> wrote:
>> Hi,
>> As part of an European (FP7) project, named ORBIT
>> (http://www.orbitproject.eu/), I'm working on including the possibility of
>> live-migrating VMs in OpenStack in a post-copy mode.
>> This way of live-migrating VMs basically moves the computation right away to
>> the destination and then the VM starts working from there, while still
>> copying the memory from the source to the new location of the VM. That way
>> the memory pages are only copied as if the VM modifies them, they are
>> already in the destination host. This basically ensures that migrations
>> finish regardless of what the VM is doing, i.e., even extremely memory
>> intensive VMs. Therefore removing the problem of having VMs hanging on in
>> migrating state forever (as discussed in previous mails, e.g.,
>> http://lists.openstack.org/pipermail/openstack-dev/2015-February/055725.html).
>> So far, I have included and tested this new functionality at the JUNO
>> version, and the code modifications can be found in the github repository of
>> the project (branch named "post-copy"):
>>      - https://github.com/orbitfp7/nova/tree/post-copy --> mainly enabling
>> the possibility of using the libvirt post-copy flag (libvirt driver.py).
>> Note post-copy migration is not using "tunneling" as LibVirt patch for that
>> is not yet ready.
>>      - https://github.com/orbitfp7/python-novaclient/tree/post-copy -->
>> adding the possibility of using the post-copy mode when triggering the
>> migration: nova live-migration [--block-migrate] [--post-copy] VM_ID
>>      - https://github.com/orbitfp7/horizon/tree/post-copy --> include a
>> checkbox in the live-migration panel to perform the migration in post-copy
>> mode. (like the one for enabling block-migration)
>> To be able to live-migrate VMs in a post-copy way, I'm relying on some
>> kernel+qemu+libvirt modifications, not yet merged upstream (but in their way
>> to it), also available at the project github:
>>      - Kernel: https://lkml.org/lkml/2015/3/5/576
>>      - Qemu: https://github.com/orbitfp7/qemu/tree/wp3-postcopy
>>      - LibVirt: https://github.com/orbitfp7/libvirt/tree/wp3-postcopy
> Before merging the code in Nova, we usually like the dependent
> features to be released by the respective projects.
> Ideally we would like it to be easy to run that on some distro so
> people could test/use the feature fairly easily.
Yes, that's why I proposed to target the version after kilo (or even the 
next to that one if need be)

>> If this is a nice feature to have in future versions of OpenStack, I'm happy
>> to adapt the code for the next release (the one after KILO). Any comments
>> are really welcome.
> It sounds like something that doesn't need an API call, as its a
> deployer choice if they have support for this new live-migrate mode.
> Is that true?
> Although maybe it has a substantial runtime penalty as a page read
> miss causes a fetch across the network, making it a user choice? Or do
> you only start the fetch mode at the point you detect a failure to
> "merge" using the regular live-migrate mode?

I think it should be up to the user/admin what option to choose.
Although post-copy ensures that the migration will finish, as you said, 
it could have some impact into the VM performance due to having to wait 
until a missing memory page is fetched. Anyway, I wouldn't say there is 
a substantial runtime penalty. In fact, the libvirt flag that we have 
included in OpenStack basically tries pre-copy first (normal 
live-migration), and after trying to copy all the memory once (first 
iteration), automatically changes to post-copy, meaning moving the VM 
cpu to the destination and only having to copy the remaining pages (the 
ones dirtied while doing the first copy iteration). This way the impact 
into the application performance is minimized.

On the other hand, post-copy has a downside. If by any chance the 
migration crash during the process, unlike pre-copy, you can not recover 
the VM as not the source nor the destination has a fully working VM at 
the time (part of the memory in the source, part of it at the destination).

These are basically the reasons we considered for making it as an 
optional choice.

Thank you for your comments!

> Thanks,
> John
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Dr. Luis Tomás
Postdoctoral Researcher
Department of Computing Science
Umeå University
luis at cs.umu.se

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