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

[Anthony Liguori]

On 06/04/2012 05:57 PM, Isaku Yamahata wrote:
> After the long time, we have v2. This is qemu part.
> The linux kernel part is sent separatedly.
>
> Changes v1 ->  v2:
> - split up patches for review
> - buffered file refactored
> - many bug fixes
>    Espcially PV drivers can work with postcopy
> - optimization/heuristic
>
> Patches
> 1 - 30: refactoring exsiting code and preparation
> 31 - 37: implement postcopy itself (essential part)
> 38 - 41: some optimization/heuristic for postcopy
>
> 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] [-m]] URI [<prefault forward>  [<prefault backword>]]
>    -p: indicate postcopy migration
>    -n: disable background transferring pages: This is for benchmark/debugging
>    -m: move background transfer of postcopy mode
>    <prefault forward>: The number of forward pages which is sent with on-demand
>    <prefault backward>: The number of backward pages which is sent with
>                         on-demand
>
>    example:
>    migrate -p -n tcp:<dest ip address>:4444
>    migrate -p -n -m tcp:<dest ip address>:4444 32 0
>
>
> 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.

Regards,

Anthony Liguori