Re: [Qemu-devel] [PATCH v2 00/41] postcopy live migration

[Juan Quintela]

Anthony Liguori <address@hidden> wrote:
>> TODO
>> ====
>> - benchmark/evaluation. Especially how async page fault affects the result.
>
> I don't mean to beat on a dead horse, but I really don't understand
> the point of postcopy migration other than the fact that it's
> possible.  It's a lot of code and a new ABI in an area where we
> already have too much difficulty maintaining our ABI.
>
> Without a compelling real world case with supporting benchmarks for
> why we need postcopy and cannot improve precopy, I'm against merging
> this.

I understand easily the need/want for post-copy migration.  Other thing
is that this didn't came with benchmarks and that post-copy is
difficult.

The basic problem with precopy is that the amount of memory used by
guest is not going to go down any time soon.  The same with number of
cores.  At some point (it didn't matter if it is 16GB, 128GB or 256GB
RAM in the guest, the same for vcpus), precopy just don't have a chance.
And post-copy does.

Once told that, we need to measure what is the time of an async page
fault over the network.  If it is too high, post copy just don't work.

And no, I haven't seen any measurement that told us that this is going
to be fast enough, but there is always hope.

Later, Juan.