Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Paolo Bonzini <pbonzini@redhat.com>]

Il 18/03/2013 18:38, George Dunlap ha scritto:
>>>
>> This might be a difference between Xen and KVM. On Xen migration is
>> made to a server in a paused state, and it's only unpaused when
>> the migration to B is complete. There's a sort of extra handshake at
>> the end.
> 
> I think what you mean is that all the memory is handled by Xen and the
> toolstack, not by qemu.  The qemu state is sent as the very last thing,
> after all of the memory, and therefore (you are arguing) that qemu is
> not started, and the files cannot be opened, until after the migration
> is nearly complete, and certainly until after the file is closed on the
> sending side.

That would be quite dangerous.  Files aren't closed until after QEMU
exits; at this point whatever problem you have launching QEMU on the
destination would be unrecoverable.

Even for successful migration, it would also be bad for downtime (QEMU
isn't exactly lightning-fast to start).  And even if failure weren't
catastrophic, it would be a pity to transfer a few gigs of memory and
then find out that QEMU isn't present in the destination. :)

Still, it's more than possible that I've forgotten something about Xen's
management of QEMU.

> (In KVM this isn't the case because I assume it's qemu that handles the
> memory transition, and so it is likely to open the files on the
> receiving side when the migration starts.)

Yup.

Paolo