Re: [Qemu-devel] [PATCH] migration: adding migration to/from a file

[Uri Lublin <uril@redhat.com>]

Daniel P. Berrange wrote:
> On Mon, Jan 19, 2009 at 09:11:28AM -0600, Anthony Liguori wrote:
>> Uri Lublin wrote:
>>> Migration to file, reuses migration-to-fd.
>>> Migration from file, uses qemu-fopen directly.
>>>
>>> The saved state-file should be used only once and removed (or used
>>> with -snapshot, or a the disk-image should be copied), as the
>>> disk image is not saved, only the VM state.
>>>
>>> Also there is not point of doing a _live_ migration to file (except
>>> for debugging migration code), so I recommend to stop the VM before 
>>> migrating its state to a file.
>>>
>>> An advantage migration-to-file over savevm/loadvm is that for the latter
>>> a qcow2 is a requirement, while the former works for any image-format.
>>>
>>> Signed-off-by: Uri Lublin <uril@redhat.com>
>>> ---
>>> 

>> The migration code assumes that the file descriptor used is 
>> non-blocking.  In general, open() on a file system cannot produce a 
>> non-blocking file descriptor.
> 
> Does it matter if it blocks though ?  Migrating to a file is not
> "live" migration anyway, so if we happen to block the rest of the 
> QEMU event loop during course of writing to the file it doesn't seem
> too serious. Or is there some scenario which badly breaks with blocking
> writes even for non-live usage ?
> 

I first thought it's not that important too.
But it may be less than optimal, especially if qemu does not respond to monitor 
commands, which may affect management applications.
What does libvirt do when qemu does not respond to a monitor command ?

Uri.