Re: [Qemu-devel] [PATCH] XBRLE page delta compression for live migration of large memory apps

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Jun 22, 2011 at 1:01 PM, Anthony Liguori <anthony@codemonkey.ws> wrote:
> On 05/22/2011 07:00 AM, Shribman, Aidan wrote:
>>
>> Subject: [PATCH] XBRLE page delta compression for live migration of large
>> memory apps
>> From: Aidan Shribman<aidan.shribman@sap.com>
>>
>> By using XBRLE (Xor Based Run-Length-Encoding) we can reduce required
>> bandwidth for transfering of dirty memory pages during live migration
>>         migrate_set_cachesize<size>
>>         migrate -x<url>
>> Qemu host: Ubuntu 10.10
>> Testing: live migration (w and w/o XBRLE) tested successfully.
>
> By how much?
>
> This is a change to the live migration protocol, it would also require
> documentation and an understanding of how it affects compatibility.
>
> The patch really needs to be split into logical pieces too.  It's a bit too
> big for a meaningful review.

Two places where you could consider splitting the patch is the caching
and the sampling.  Are they necessary for correctness and could they
be submitted as follow-up patches to a core patch which does just the
XBRLE?

Also, whenever there are heuristics and use of floating point then
there is some magic going on.  It may be necessary and give a huge
performance boost but needs explanation so it is not a black box or
fragile mechanism once it has been merged upstream.

Stefan