Re: [Qemu-devel] [PATCH 00/21][RFC] postcopy live migration

[Anthony Liguori <anthony@codemonkey.ws>]

On 12/28/2011 07:25 PM, Isaku Yamahata wrote:
> Intro
> =====
> This patch series implements postcopy live migration.[1]
> As discussed at KVM forum 2011, dedicated character device is used for
> distributed shared memory between migration source and destination.
> Now we can discuss/benchmark/compare with precopy. I believe there are
> much rooms for improvement.
>
> [1] http://wiki.qemu.org/Features/PostCopyLiveMigration
>
>
> Usage
> =====
> You need load umem character device on the host before starting migration.
> Postcopy can be used for tcg and kvm accelarator. The implementation depend
> on only linux umem character device. But the driver dependent code is split
> into a file.
> I tested only host page size == guest page size case, but the implementation
> allows host page size != guest page size case.
>
> The following options are added with this patch series.
> - incoming part
>    command line options
>    -postcopy [-postcopy-flags<flags>]
>    where flags is for changing behavior for benchmark/debugging
>    Currently the following flags are available
>    0: default
>    1: enable touching page request
>
>    example:
>    qemu -postcopy -incoming tcp:0:4444 -monitor stdio -machine accel=kvm
>
> - outging part
>    options for migrate command
>    migrate [-p [-n]] URI
>    -p: indicate postcopy migration
>    -n: disable background transferring pages: This is for benchmark/debugging
>
>    example:
>    migrate -p -n tcp:<dest ip address>:4444
>
>
> TODO
> ====
> - benchmark/evaluation. Especially how async page fault affects the result.

I'll review this series next week (Mike/Juan, please also review when you can).

But we really need to think hard about whether this is the right thing to take 
into the tree.  I worry a lot about the fact that we don't test pre-copy 
migration nearly enough and adding a second form just introduces more things to 
test.

It's also not clear to me why post-copy is better.  If you were going to sit 
down and explain to someone building a management tool when they should use 
pre-copy and when they should use post-copy, what would you tell them?

Regards,

Anthony Liguori