[CentOS-virt] The future of blktap in Xen4CentOS

Fri May 8 22:10:36 UTC 2015
Johnny Hughes <johnny at centos.org>

On 05/06/2015 09:27 AM, George Dunlap wrote:
> 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.

I did this with the kernel I released .. the blktap module is loaded
upon boot.. I did have to change some of the code based on this Debain bug:


I have no idea how well this works .. we could use some testing of that
kernel here:


> 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).

This has not been a huge problem in the past .. it mostly works OK.  At
least no one has complained.  The only time I had to change it was when
we moved from 3.6 to 3.10 and then from 3.10 to 3.18.  But that does not
mean it works without issue, just that no one complained about it.

> 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.

I have no real preference .. just that we need a kernel update in 3
months as in Sept 2015, 3.10 support stops at kernel.org.

We can use the 3.18 LTS tree with my mods now, if it works for everyone
.. although the best way longterm seems to be to use blktap3 in the
future since that is what is in XenServer.

Johnny hughes

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