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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 00/21][RFC] postcopy live migration
Date: Thu, 29 Dec 2011 16:39:52 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15

On 12/28/2011 07:25 PM, Isaku Yamahata wrote:
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

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

   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

   migrate -p -n tcp:<dest ip address>:4444

- 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?


Anthony Liguori

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