Re: [Qemu-devel] [RFC PATCH v2 00/23] COarse-grain LOck-stepping(COLO) Virtual Machines for Non-stop Service

[Dr. David Alan Gilbert]

* Wen Congyang (address@hidden) wrote:

<snip>

> Hi all:
> 
> I will start to implement disk replication. Before doing this, I think we 
> should decide
> how to implement it.
> 
> I have two ideas about it:
> 1. implement it in qemu
>    Advantage: very easy, and don't take too much time
>    Disadvantage: the virtio disk with vhost is not supported, because the 
> disk I/O
>        operations are not handled in qemu.
> 
> 2. update drbd and make it support colo
>    Advantage: we can use it for both KVM and XEN.
>    Disadvantage: The implementation may be complex, and need too much time to
>         implement it.(I don't read the drbd's codes, and can't estimate the 
> cost)
> 
> I think we can use 1 to implement it first.
> If you have some other idea, please let me know.

For the COLO disk replication; are you talking here about 'local storage'
and treating it as 'internal state' or 'external state' (as described in the
first half of 4.4 in the original COLO paper)?

I know some groups would like to take advantage of facilities in the storage
layer to help; e.g. take snapshots/rollback etc - so I think it's best
to do (1) but make the interface clean so that other mechanisms could be added.
Similarly I guess things like scsi-xcopy might help for some people - I'm
not saying implement these, just if possible make an interface where it could
fit later.

It's probably best to check with the QEMU storage guys that you can reuse
anything they have; there was a discussion a few weeks back where I cc'd
Fam, Stefan and Kevin in).

Dave

--
Dr. David Alan Gilbert / address@hidden / Manchester, UK