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

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On 08/08/2011 06:29 PM, Anthony Liguori wrote:

- Efficient, reduce needed traffic no need to re-send pages.

It's not quite that simple. Post-copy needs to introduce a protocol
capable of requesting pages.

Just another subsection.. (kidding), still it shouldn't be too
complicated, just an offset+pagesize and return page_content/error

What I meant by this is that there is potentially a lot of round trip overhead. Pre-copy migration works well with reasonable high latency network connections because the downtime is capped only by the maximum latency sending from one point to another.

But with something like this, the total downtime is 2*max_latency*nb_pagefaults. That's potentially pretty high.

Let's be generous and assume that the latency is dominated by page copy time. So the total downtime is equal to the first live migration pass, ~20 sec for 2GB on 1GbE. It's distributed over potentially even more time, though. If the guest does a lot of I/O, it may not be noticeable (esp. if we don't copy over pages read from disk). If the guest is cpu/memory bound, it'll probably suck badly.


So it may be desirable to try to reduce nb_pagefaults by prefaulting in pages, etc. Suffice to say, this ends up getting complicated and may end up burning network traffic too.

Yeah, and prefaulting in the background adds latency to synchronous requests.

This really needs excellent networking resources to work well.

--
error compiling committee.c: too many arguments to function

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