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

Richard W.M. Jones rjones at redhat.com
Thu Jul 19 17:03:29 UTC 2007


Some observations about Xen migration and error handling.

The Xen migration protocol isn't stable between releases.  It changed 
between 3.0.3 and 3.1.0.  There doesn't seem to be any versioning, and 
incompatible versions of Xen seem happy to attempt migrations between 
them, even though these will certainly fail.

The source host's xend forks an xc_save process.  It appears to me that 
xc_save will happily write to _anything_ listening on port 8002, even if 
that thing closes the socket prematurely.  (Try running 'xm migrate' on 
one host and at the same time 'nc -l 8002 > /dev/null' on the target 
host.  The 'xm migrate' will happily complete without error.  Meanwhile 
the domain you "migrated" just gets deleted.)

Partly because of the lack of error reporting, and partly because the 
xend -> xc_save fork will make error reporting difficult to add, libvirt 
has a hard time displaying errors in this situation.  It is quite 
possible to call virDomainMigrate and get a domain back which shortly 
afterwards "disappears", all without any indication of error.

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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