[Libvir] [PATCH] virDomainMigrate version 4 (for discussion only!)

[Richard W.M. Jones]

This is version 4 of the virDomainMigrate patch.  It includes remote 
support, but no qemu, virsh or python yet.  And it needs lots more 
testing which I intend to do more of tomorrow.

Interface
---------

The interface is now this:

virDomainPtr
   virDomainMigrate (virDomainPtr domain, virConnectPtr dconn,
                     unsigned long flags, const char *dname,
                     const char *uri, unsigned long resource);

The caller may set dname, uri and resource to 0/NULL and forget about 
them.  Or else the caller may set, in particular, uri to allow for more 
complicated migration strategies (especially for qemu).

https://www.redhat.com/archives/libvir-list/2007-July/msg00249.html

Driver support
--------------

As outlined in the diagram in this email:
https://www.redhat.com/archives/libvir-list/2007-July/msg00264.html
migration happens in two stages.

Firstly we send a "prepare" message to the destination host.  The 
destination host may reply with a cookie.  It may also suggest a URI (in 
the current Xen implementation it just returns gethostname).  Secondly 
we send a "perform" message to the source host.

Correspondingly, there are two new driver API functions:

   typedef int
       (*virDrvDomainMigratePrepare)
                       (virConnectPtr dconn,
                        char **cookie,
                        int *cookielen,
                        const char *uri_in,
                        char **uri_out,
                        unsigned long flags,
                        const char *dname,
                        unsigned long resource);

   typedef int
       (*virDrvDomainMigratePerform)
                       (virDomainPtr domain,
                        const char *cookie,
                        int cookielen,
                        const char *uri,
                        unsigned long flags,
                        const char *dname,
                        unsigned long resource);

Remote support
--------------

To make this work in the remote case I have had to export two private 
functions from the API which only remote should call:
   __virDomainMigratePrepare
   __virDomainMigratePerform
The reason for this is that libvirtd is just linked to the regular 
libvirt.so, so can only make calls into libvirt through exported symbols 
in the dynamic symbol table.

There are two corresponding wire messages 
(REMOTE_PROC_DOMAIN_MIGRATE_PREPARE and 
REMOTE_PROC_DOMAIN_MIGRATE_PERFORM) but they just do dumb argument 
shuffling, albeit rather complicated because of the number of arguments 
passed in and out.

The complete list of messages which go across the wire during a 
migration is:

   client -- prepare --> destination host
   client <-- prepare reply -- destination host
   client -- perform --> source host
   client <-- perform reply -- source host
   client -- lookupbyname --> destination host
   client <-- lookupbyname reply -- destination host

Xen URIs
--------

Xen recognises the following forms of URI:
   hostname
   hostname:port
   tcp://hostname/
   tcp://hostname:port/

Capabilities
------------

I have extended capabilities with <migration_features>.  For Xen this is:

<capabilities>
   <host>
    <migration_features>
      <live/>
      <uri_transports>
        <uri_transport>tcp</uri_transport>
      </uri_transports>
    </migration_features>

Rich.

-- 
Emerging Technologies, Red Hat - http://et.redhat.com/~rjones/
Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod
Street, Windsor, Berkshire, SL4 1TE, United Kingdom.  Registered in
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