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Re: [Xen-devel] libvirt, libxl and QDISKs

On Fri, 26 Apr 2013, 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.

If it is safe for QEMU, it should be safe for loop and blkback too.

> > 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...

Yes, libxl would need to arrage the script to be called when "phy" is
used on a file, right?

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