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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 26 Apr 2013, Ian Campbell wrote:
> On Fri, 2013-04-26 at 12:48 +0100, Stefano Stabellini wrote:
> > 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.
> 
> qemu (and blkback) will issue correct barriers/syncs to the underlying
> storage. AFAIK loop does not.

Looking at drivers/block/loop.c, it seems to me that REQ_FUA and
REQ_FLUSH both handled correctly by issuing a vfs_fsync.


> > > > 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?
> 
> I meant the user can pass "script=block-loop". In the full knowledge of
> what that means for their data integrity.
 
Yeah, but there is no such block-loop script at the moment, the script
is called block and its behaviour depends on the xenstore "type" node:
it must be "file" and libxl never writes "type" "file" to xenstore (see
device_disk_add and libxl__device_disk_string_of_backend).

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