Re: [Qemu-devel] [PATCH 3/10] Add bdrv_flush_all()

[Anthony Liguori <aliguori@us.ibm.com>]

Glauber Costa wrote:
> On Wed, Sep 10, 2008 at 10:26 AM, Chris Lalancette <clalance@redhat.com> wrote:
>   
>> Anthony Liguori wrote:
>>     
>>> This patch adds a bdrv_flush_all() function.  It's necessary to ensure that all
>>> IO operations have been flushed to disk before completely a live migration.
>>>
>>> N.B. we don't actually use this now.  We really should flush the block drivers
>>> using an live savevm callback to avoid unnecessary guest down time.
>>>       
>> Simple enough, and follows the pattern in the KVM migration.
>>
>> Chris Lalancette
>>
>>     
> sounds ok.
>   

I'm actually liking bdrv_flush_all() less and less.  If there are any 
outstanding IO requests, it will increase the down time associated with 
live migration.  I think we definitely need to add a live save handler 
that waits until there are no outstanding IO requests to converge.  I'm 
concerned though that it's not totally unreasonable to expect a guest to 
always have an IO request in flight.  That leads me to think that maybe 
we should be cancelling outstanding requests, and somehow saving their 
state?

Regards,

Anthony Liguori