The Future Of Blktap In Xen4CentOS

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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.

-George

3 thoughts on - The Future Of Blktap In Xen4CentOS

  • Hello,

    How badly has the blktap 2.5 kernel code bitrotted? What areas are the most problematic?
    Any idea how much work would it be to get it working with 3.18 LTS ?

    Yeah.. likely more work than the “deep port” of blktap 2.5 kernel code..

    But not very nice..

    Thanks,

    — Pasi

  • 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:

    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748953

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

    http://people.CentOS.org/hughesjr/xen-kernel/

    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.

    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.

    Thanks, Johnny hughes

  • Right — done by none other than Citrix’s own Bob Ball… who updated it in his own github branch but then forgot to send a pull request to to the main XenServer repo, which is why I didn’t see it. :-)

    Right… well I guess there are other users of the same code (which is why Bob submitted the fix last year), so we can hope that it’s at least getting a little testing. If we give it a month of beta testing before releasing it, I think that’s the best people can expect from a community-run project.

    Yes, using the forward-ported kernel blktap driver is probably the best thing to do until we have a replacement.

    -George