Re: libvirt, libxl and QDISKs

[Marek Marczykowski <marmarek@invisiblethingslab.com>]

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On 26.04.2013 13:40, Ian Campbell wrote:
> On Fri, 2013-04-26 at 12:31 +0100, Marek Marczykowski wrote:
>> What about old good loop+phy based backend for file disk images? I don't want
>> whole qemu in dom0 for PV domains, only for handling simple disk backend.
>> Additionally sparse images + loop + phy + mount -o discard in domU makes the
>> images "auto shrinking". Don't know if qemu is able to do this.
> 
> IIRC the problem with loop+phy is that loop doesn't do O_DIRECT and
> therefore your data isn't actually on the disk when you might think it
> is, which can lead to filesystem corruption even if the f/s is doing
> correct barriers.
> 
>> Attached patch, which I currently use for that. If it is close to something
>> that would be accepted, I will send it in new thread.
> 
> I think you can use a block script for this (i.e. it does the loop
> mount) and avoid patching libxl at all. That's what xend did at least...

This also was solution I've evaluated, but libvirt developers don't want to
hear about disk->script setting in official libvirt, so I still need to patch
some library. Choosing libxl seems to be more universal. Setting loop in libxl
is also much faster than calling script for that.
Perhaps I can rewrite this patch to allow the choice (LIBXL_DISK_BACKEND_LOOP)?

-- 
Best Regards / Pozdrawiam,
Marek Marczykowski
Invisible Things Lab


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