[Qemu-devel] [migration] questions about removing the old block-migration code

[Qemu-devel] [migration] questions about removing the old block-migration code

[Zhanghaoyu (A)]

Hi, Juan

I read below words on the report of <KVM Live Migration: Weather forecast (May 29, 2013)>,
We were going to remove the old block-migration code
Then people fixed it
Good: it works now
Bad: We have to maintain both
It uses the same port than migration
You need to migrate all/none of block devices

The old block-migration code said above is that in block-migration.c?
What are the reasons of removing the old block-migration code? Buggy implementation? Or need to migrate all/none of block devices?
What's the substitutional method? drive_mirror?

Thanks,
Zhang Haoyu

Re: [Qemu-devel] [migration] questions about removing the old block-migration code

[Stefan Hajnoczi]

On Sun, Nov 03, 2013 at 04:01:36AM +0000, Zhanghaoyu (A) wrote:
> I read below words on the report of <KVM Live Migration: Weather forecast (May 29, 2013)>,
> We were going to remove the old block-migration code
> Then people fixed it
> Good: it works now
> Bad: We have to maintain both
> It uses the same port than migration
> You need to migrate all/none of block devices
> 
> The old block-migration code said above is that in block-migration.c?

Yes.

> What are the reasons of removing the old block-migration code? Buggy implementation? Or need to migrate all/none of block devices?

Buggy and tightly coupled with the live migration code, making it hard
to modify either area independently.

> What's the substitutional method? drive_mirror?

drive_mirror over NBD is an alternative.  There are security and
integration challenges with those approaches but libvirt has added
drive-mirror block migration support.

Stefan

Re: [Qemu-devel] [migration] questions about removing the old block-migration code

[Zhanghaoyu (A)]

>> I read below words on the report of <KVM Live Migration: Weather 
>> forecast (May 29, 2013)>, We were going to remove the old 
>> block-migration code Then people fixed it
>> Good: it works now
>> Bad: We have to maintain both
>> It uses the same port than migration
>> You need to migrate all/none of block devices
>> 
>> The old block-migration code said above is that in block-migration.c?
>
>Yes.
>
>> What are the reasons of removing the old block-migration code? Buggy implementation? Or need to migrate all/none of block devices?
>
>Buggy and tightly coupled with the live migration code, making it hard to modify either area independently.

Thanks a lot for explaining.
Till now, we still use the old block-migration code in our virtualization solution.
Could you detail the bugs that the old block-migration code have?

Thanks,
Zhang Haoyu

>
>> What's the substitutional method? drive_mirror?
>
>drive_mirror over NBD is an alternative.  There are security and integration challenges with those approaches but libvirt has added drive-mirror block migration support.
>
>Stefan

Re: [Qemu-devel] [migration] questions about removing the old block-migration code

[Stefan Hajnoczi]

On Fri, Nov 08, 2013 at 01:17:17AM +0000, Zhanghaoyu (A) wrote:
> >> I read below words on the report of <KVM Live Migration: Weather 
> >> forecast (May 29, 2013)>, We were going to remove the old 
> >> block-migration code Then people fixed it
> >> Good: it works now
> >> Bad: We have to maintain both
> >> It uses the same port than migration
> >> You need to migrate all/none of block devices
> >> 
> >> The old block-migration code said above is that in block-migration.c?
> >
> >Yes.
> >
> >> What are the reasons of removing the old block-migration code? Buggy implementation? Or need to migrate all/none of block devices?
> >
> >Buggy and tightly coupled with the live migration code, making it hard to modify either area independently.
> 
> Thanks a lot for explaining.
> Till now, we still use the old block-migration code in our virtualization solution.
> Could you detail the bugs that the old block-migration code have?

Please use git log block-migration.c to see bugs that have been fixed.

The basic problem is that block-migration.c hasn't been actively
maintained or used much.  My impression is that the newer block
migration approach using drive-mirror now has more developer and testing
focus (see libvirt if you're curious how drive-mirror can be
orchestrated).

Stefan