On Thu, 2011-10-27 at 15:04 +0100, Alex Bligh wrote:
> Ian,
>
> --On 27 October 2011 14:44:49 +0100 Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> wrote:
>
>
> >> We do not see blktapctrl starting in Xen 4.1.1 dom0, but it does
> >> start in Xen3.
> >
> > Are you changing dom0 kernel as well as hypervisor when you say "3" and
> > "4.1.1" or is the dom0 kernel constant?
>
> No, the Xen3 kernel is 2.6.18 Centos, and the Xen 4 kernel is 3.0
> Ubuntu Oneiric.
OK, that is important information now that Xen is not supplied with a
particular kernel.
> >> In fact as far as I can tell the Ubuntu Xen4 package
> >> does not contain blktapctrl at all (which would explain why it doesn't
> >> start). Do we need this?
> >
> > AFAIK tap:aio: usually requires blktap (both kernel side and userspace).
> > There is also a block backend in qemu which can be by xl used under some
> > circumstances when blktap is not available but the same is not true of
> > xend.
>
> We are using xl (though have tried xend).
Good to know.
> > XCP (from whence that blktap version comes) is only developed/test using
> > 32 bit. Taht's not to say 64 bit doesn't work, but it might not.
>
> OK. The idea here is a production xen4.1 system, so "might not work"
> sounds like a bad idea :-)
>
> >> Isn't this what blkback does?
> >
> > blkback can only export raw block devices to guests.
>
> We will in production use raw block devices, so that's ok. We were
> just using files for testing. Is the same restriction there is
> Xen3.3, because it works there (possibly for the reason below).
The 2.6.18 kernel has blktap.ko and 3.0 does not, which is going to be a
big factor. blkback.ko is in both so you should be ok. For testing you
could try using loopback like Stefano suggests.
> > A raw image can
> > also be mounted with a loop back device which itself can be exported via
> > blkback. Some toolstacks (xend) can do this automatically (but won't for
> > a tap:aio: AFAIK) but xl will not.
>
> I think xl /must/ be doing this, or it is difficult to explain the
> behaviour below.
Indeed, I can't explain it either. I think I would expect xl to fall
back to the qemu based backend if blktap wasn't available. I _think_
that functionality was backported to the qemu-xen in 4.1, but I'm not
sure.
> We discovered Ubuntu's Xen 4.1 hypervisor package does not contain
> blktapctrl, so we copied that across from a build from source, plus some
> libraries, and did an mknod on /dev/xen/blktap0 (which it appeared to want
> to open). pvdrivers then appeared in the guest. But we then reversed
> everything we'd done (we think), and pvdrivers continued to appear. We are
> reinstalling to find out exactly what it was that changed.
>
> I must admit to remaining almost totally confused as to how this is
> meant to work, but thanks for your help thus far!
>
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