[CentOS-virt] The future of blktap in Xen4CentOS

Wed May 6 14:27:54 UTC 2015
George Dunlap <dunlapg at umich.edu>

On Tue, May 5, 2015 at 2:07 PM, Johnny Hughes <johnny at centos.org> wrote:
> WRT a new kernel .. the 3.18.x kernel has been selected to be maintained
> as an LTS until Jan 2017.  I think we should start work on rebuilding
> this kernel and using it in the x4c projects.
> https://lwn.net/Articles/636289/
> Thoughts?
> I will grab this kernel and try to roll in all the patches, etc. and see
> if I can make it work.

I did a little bit of work on building the 3.18 kernel a couple of
weeks ago, but it looks like the blktap2.5 kernel code we were using
before has totally bitrotted.

I see three ways forward:

1. Try to do a "deep port" of the functionality, continuing to
maintain it going forward.

2. Start using the blktap3 code, which requires no kernel driver.

3. Drop blktap support, and ask people to use qdisk for vhd.

Re #1, I'm not really excited about the idea of trying to keep porting
those patches forward (and finding and fixing bugs in the forward-port
each time).

Re #2, this would require getting the blktap3 stuff working upstream
and then backporting it.

#3 would be by far the easiest.

In parallel to the Xen for C7 work I've been doing, I've been working
on getting upstream Xen to build against XenServer's blktap code, with
then aim of moving to blktap3.  If we want to go with #2, then a major
kernel update will have to wait until that work is done.