Re: [Qemu-devel] [RFC] live snapshot, live merge, live block migration

[Dor Laor <dlaor@redhat.com>]

On 05/13/2011 06:16 AM, Jagane Sundar wrote:
> On 5/12/2011 8:33 AM, Jes Sorensen wrote:
>> On 05/09/11 15:40, Dor Laor wrote:
>>> Summary:
>>> * We need Marcelo's new (to come) block copy implementation
>>> * should work in parallel to migration and hotplug
>>> * General copy on read is desirable
>>> * Live snapshot merge to be implemented using block copy
>>> * Need to utilize a remote block access protocol (iscsi/nbd/other)
>>> Which one is the best?
>>> * Keep qemu-img the single interface for dirty block mappings.
>>> * Live block migration pre copy == live copy + block access protocol
>>> + live migration
>>> * Live block migration post copy == live migration + block access
>>> protocol/copy on read.
>>>
>>> Comments?
>> I think we should add Jagane Sundar's Livebackup to the watch list here.
>> It looks very interesting as an alternative way to reach some of the
>> same goals.
>>
>> Cheers,
>> Jes
> Thanks for the intro, Jes. I am very interested in garnering support for
> Livebackup.
>
> You are correct in that Livebackup solves some, but not all, problems in
> the same space.
>
> Some comments about my code: It took me about two months of development
> before I connected with you on the list.
> Initially, I started off by doing a dynamic block transfer such that
> fewer and fewer blocks are dirty till there are no more dirty blocks and
> we declare the backup complete. The problem with this approach was that
> there was no real way to plug in a guest file system quiesce function. I
> then moved on to the snapshot technique. With this snapshot technique I
> am also able to test the livebackup function very thoroughly - I use a
> technique where I create a LVM snapshot simultaneously, and do a cmp of
> the LVM snapshot and the livebackup backup image.
>
> With this mode of testing, I am very confident of the integrity of my
> solution.
>
> I chose to invent a new protocol that is very simple, and custom to
> livebackup, because I needed livebackup specific functions such as
> 'create snapshot', 'delete snapshot', etc. Also, I am currently
> implementing SSL based encryption with both client authenticating to
> server and server authenticating to client using self signed certificate.
> iSCSI or NBD would be more standards compliant, I suppose.

+1 that iScsi/NBD have better potential.

>
> My high level goal is to make this a natural solution for Infrastructure
> As A Cloud environments. I am looking carefully at integrating the
> management of the Livebackup function into Openstack.

One important advantage of live snapshot over live backup is support of 
multiple (consecutive) live snapshots while there can be only a single 
live backup at one time.

This is why I tend to think that although live backup carry some benefit 
(no merge required), the live snapshot + live merge are more robust 
mechanism.

>
> I would like to help in any way I can to make KVM be the *best* VM
> technology for IaaS clouds.

:)

>
> Thanks,
> Jagane
>
>
>
>