Re: [RFC Patch 00/25] COarse-grain LOck-stepping Virtual Machines for Non-stop Service

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/07/14 12:38, Wen Congyang wrote:
> Virtual machine (VM) replication is a well known technique for providing
> application-agnostic software-implemented hardware fault tolerance -
> "non-stop service". Currently, remus provides this function, but it buffers
> all output packets, and the latency is unacceptable.
>
> In xen summit 2012, We introduce a new VM replication solution: colo
> (COarse-grain LOck-stepping virtual machine). The presentation is in
> the following URL:
> http://www.slideshare.net/xen_com_mgr/colo-coarsegrain-lockstepping-virtual-machines-for-nonstop-service
>
> Here is the summary of the solution:
> >From the client's point of view, as long as the client observes identical
> responses from the primary and secondary VMs, according to the service
> semantics, then the secondary vm is a valid replica of the primary
> vm, and can successfully take over when a hardware failure of the
> primary vm is detected.
>
> This patchset is RFC, and implements the frame of colo:
> 1. Both primary vm and secondary vm are running
> 2. do checkoint
>
> This patchset is based on remus-v15, and use migration v1. Only supports hvm
> guest now.
>
> TODO list:
> 1. rebase to remus-v17 or newer
> 2. support migration v2
> 3. nic/disk replication
> 4. support pvm

This is an interesting set of patches.  One query I have is what you
mean by "support migration v2".

While I am developing migration v2, the old legacy code is left in place
for easier testing and development purposes, but when the migration v2
code does finally get committed, the legacy migration code will be
deleted. 

The legacy code is unfit for purpose and entirely replaced by v2, in a
backwards-compatible manor given the included conversion script (not
that that matters for Remus/colo).

~Andrew