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Re: [libvirt] [PATCH V6] support offline migration



On Tue, Sep 11, 2012 at 08:30:08AM -0600, Eric Blake wrote:
> On 09/10/2012 08:20 PM, liguang wrote:
> > original migration did not aware of offline case
> > so, add code to support offline migration quietly
> > (did not disturb original migration) by pass
> > VIR_MIGRATE_OFFLINE flag to migration APIs if the
> > domain is really inactive, and
> > migration process will not puzzeled by domain
> 
> s/puzzeled/puzzled/
> 
> > offline and exit unexpectly.
> 
> s/unexpectly/unexpectedly/
> 
> > these changes did not take care of disk images the
> > domain required, for disk images could be transfered
> 
> s/transfered/transferred/
> 
> > by other APIs as suggested.
> > so, the migration result is just make domain
> > definition alive at target side.
> > 
> > Signed-off-by: liguang <lig fnst cn fujitsu com>
> > ---
> >  include/libvirt/libvirt.h.in |    1 +
> >  src/qemu/qemu_driver.c       |    8 ++++++
> >  src/qemu/qemu_migration.c    |   55 ++++++++++++++++++++++++++++++++++++-----
> >  src/qemu/qemu_migration.h    |    3 +-
> >  tools/virsh-domain.c         |    6 ++++
> >  5 files changed, 65 insertions(+), 8 deletions(-)
> > 
> > diff --git a/include/libvirt/libvirt.h.in b/include/libvirt/libvirt.h.in
> > index cfe5047..77df2ab 100644
> > --- a/include/libvirt/libvirt.h.in
> > +++ b/include/libvirt/libvirt.h.in
> > @@ -995,6 +995,7 @@ typedef enum {
> >                                                 * whole migration process; this will be used automatically
> >                                                 * when supported */
> >      VIR_MIGRATE_UNSAFE            = (1 << 9), /* force migration even if it is considered unsafe */
> > +    VIR_MIGRATE_OFFLINE           = (1 << 10), /* offline migrate */
> 
> Do we really need a new user-visible flag, or can we make this work
> automatically without having to involve the user?
> On the other hand, what happens if we do keep this as a user-visible
> flag?  Should 'virsh migrate --offline' silently ignore the flag if the
> guest is online, or should it error out stating that the guest is
> running and not offline?
> 
> Also, I think we NEED to error out if the guest is offline but the
> --persistent flag is not set; that is, an offline migration only makes
> sense if the persistent flag has been requested, but I think that 'virsh
> migrate --persistent' should automatically be smart enough to do an
> offline migration.

No we must not do that. If a guest has shutoff we cannot assume that
the user / app wants to copy it across to the other host. eg consider
this scenario

  admin a: check if guestfoo is running
  admin b: check if guestfoo is running
  admin a: migrate guestfoo barhost
  admin b: migrate guestfoo wizzhost

IMHO step 4 should fail unless the admin explicitly requested
that they want to copy across the offline config


> You either need to take care of migrating storage if the user does
> 'virsh migrate [whatever-we-decide-for-offline] --copy-storage-*', or
> else explicitly reject attempts to migrate storage in parallel with an
> offline migration.
> 


Daniel
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