[libvirt-users] live migration via unix socket

[Martin Kletzander]

On Mon, Sep 10, 2018 at 02:38:48PM -0400, David Vossel wrote:
>On Wed, Aug 29, 2018 at 4:55 AM, Daniel P. Berrangé <berrange at redhat.com>
>wrote:
>
>> On Tue, Aug 28, 2018 at 05:07:18PM -0400, David Vossel wrote:
>> > Hey,
>> >
>> > Over in KubeVirt we're investigating a use case where we'd like to
>> perform
>> > a live migration within a network namespace that does not provide
>> libvirtd
>> > with network access. In this scenario we would like to perform a live
>> > migration by proxying the migration through a unix socket to a process in
>> > another network namespace that does have network access.  That external
>> > process would live on every node in the cluster and know how to correctly
>> > route connections between libvirtds.
>> >
>> > virsh example of an attempted migration via unix socket.
>> >
>> > virsh migrate --copy-storage-all --p2p --live --xml domain.xml my-vm
>> > qemu+unix:///system?socket=destination-host-proxy-sock
>> >
>> > In this example, the src libvirtd is able to establish a connection to
>> the
>> > destination libvirtd via the unix socket proxy. However, the
>> migration-uri
>> > appears to require either tcp or rdma network connection.  If I force the
>> > migration-uri to be a unix socket, I receive an error [1] indicating that
>> > qemu+unix is not a valid transport.
>>
>> qemu+unix is a syntax for libvirt's URI format.  The URI scheme for
>> migration is not the same, so you can't simply plug in qemu+unix here.
>>
>> >
>> > Technically with qemu+kvm I believe what we're attempting should be
>> > possible (even though it is inefficient). Please correct me if I'm wrong.
>> >
>> > Is there a way to achieve this migration via unix socket functionality
>> this
>> > using Libvirt? Also, is there a reason why the migration uri is limited
>> to
>> > tcp/rdma
>>
>> Internally libvirt does exactly this when using its TUNNELLED live
>> migration
>> mode. In this QEMU is passed an anonymous UNIX socket and the data is all
>> copied over the libvirtd <-> libvirtd connection and then copied again back
>>
>
>Sorry for the delayed response here, I've only just picked this task back
>up again recently.
>
>With the TUNNELLED and PEER2PEER migration flags set, Libvirt won't allow
>the libvirtd <-> libvirtd connection over a unix socket.
>
>Libvirt returns this error "Attempt to migrate guest to the same host".
>The virDomainMigrateCheckNotLocal() function ensures that a peer2peer
>migration won't occur when the destination is a unix socket.
>
>Is there anyway around this? We'd like to tunnel the destination connection
>through a unix socket. The other side of the unix socket is a network proxy
>in a different network namespace which properly performs the remote
>connection.
>

IMHO that is there just for additional safety since the check with serves the
same purpose is done again in more sensible matter later on (checking that the
hostnames and UUIDs are different).  Actually it's just an older check before
the UUID and hostname were sent in the migration cookie.  And that's there for
quite some time.

IMHO that check can go.  In the worst case we can skip that check
(!tempuri->server) if you ask for unsafe migration.

Also, just to try it out, you *might* be able to work around that check by using
something like unix://localhost.localdomain/path/to/unix.socket (basically
adding any hostname different than localhost there), but I might be wrong there.

>
>> to QEMU on another UNIX socket. This was done because QEMU has long had no
>> ability to encrypt live migration, so tunnelling over libvirtd's own TLS
>> secured connection was only secure mechanism.
>
>
>> We've done work in QEMU to natively support TLS now so that we can get rid
>> of this tunnelling, as this architecture decreased performance and consumed
>> precious CPU memory bandwidth, which is particularly bad when libvirtd and
>> QEMU were on different NUMA nodes. It is already a challenge to get live
>> migration to successfully complete even with a direct network connection.
>> Although QEMU can do it at the low level, we've never exposed anything
>> other than direct network transports at the API level.
>>
>> Regards,
>> Daniel
>> --
>> |: https://berrange.com      -o-    https://www.flickr.com/photos/
>> dberrange :|
>> |: https://libvirt.org         -o-
>> https://fstop138.berrange.com :|
>> |: https://entangle-photo.org    -o-    https://www.instagram.com/
>> dberrange :|
>>
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