Re: [Qemu-devel] [RFC] postcopy livemigration proposal

[Anthony Liguori <anthony@codemonkey.ws>]

On 08/08/2011 11:52 AM, Cleber Rosa wrote:
> On 08/08/2011 07:47 AM, Dor Laor wrote:
>> On 08/08/2011 01:59 PM, Nadav Har'El wrote:
>>>>> * What's is postcopy livemigration
>>>>> It is is yet another live migration mechanism for Qemu/KVM, which
>>>>> implements the migration technique known as "postcopy" or "lazy"
>>>>> migration. Just after the "migrate" command is invoked, the execution
>>>>> host of a VM is instantaneously switched to a destination host.
>>>
>>> Sounds like a cool idea.
>>>
>>>>> The benefit is, total migration time is shorter because it transfer
>>>>> a page only once. On the other hand precopy may repeat sending same
>>>>> pages
>>>>> again and again because they can be dirtied.
>>>>> The switching time from the source to the destination is several
>>>>> hunderds mili seconds so that it enables quick load balancing.
>>>>> For details, please refer to the papers.
>>>
>>> While these are the obvious benefits, the possible downside (that, as
>>> always, depends on the workload) is the amount of time that the guest
>>> workload runs more slowly than usual, waiting for pages it needs to
>>> continue. There are a whole spectrum between the guest pausing
>>> completely
>>> (which would solve all the problems of migration, but is often
>>> considered
>>> unacceptible) and running at full-speed. Is it acceptable that the guest
>>> runs at 90% speed during the migration? 50%? 10%?
>>> I guess we could have nothing to lose from having both options, and
>>> choosing
>>> the most appropriate technique for each guest!
>
> Not sure if it's possible to have smart heuristics on guest memory page
> faults, but maybe a technique that reads ahead more pages if a given
> pattern is detected may help to lower the impact.

It's got to be a user choice.  Post-copy can mean unbounded downtime for 
a guest with no way to mitigate it.  It's impossible to cancel a 
post-copy migration.

I actually think the use-cases for post-copy are fairly limited in an 
enterprise environment.

Regards,

Anthony Liguori