[Libguestfs] Question about mounting QCOW2 files....

[Richard W.M. Jones]

On Mon, Oct 15, 2018 at 10:28:25AM -0400, Raghuram Devarakonda wrote:
> On Mon, Oct 15, 2018 at 3:54 AM Richard W.M. Jones <rjones at redhat.com> wrote:
> >
> > [Adding libguestfs mailing list]
> >
> > On Sun, Oct 14, 2018 at 11:53:17AM -0400, Raghuram Devarakonda wrote:
> > > Hi,
> > >
> > > In our project, we deal with quite large sparse files whose total size
> > > runs into several hundred terabytes or even more, though actual
> > > allocated size on disk is much smaller. The problem is that such large
> > > files present issues for copying around, compression, or even for
> > > computing checksums. I am wondering of I can use guestfs-tools to
> > > mount a QCOW2 image and then use the image as sparse file. The idea is
> > > that actual file on the disk would be compact though to our code,
> > > sparse file interface is preserved.
> >
> > It's not very clear to me exactly what you want, but in general yes
> > qcow2 is a good way to handle very large, sparse disk images.
> >
> > If you can be clearer about exactly what you mean by "mount" then I
> > could answer the question better.  For example, do you mean "mount a
> > filesystem in the qcow2 image"?  In which case use guestmount.  If you
> > mean "attach the qcow2 disk as a local device" then qemu-nbd can do
> > this.
> 
> We would like to create and write to a large sparse file but would
> like to keep the file on disk itself in compact format. So we are not
> talking about a file system but a single file. I guess this means
> creating qcow2 disk and attach as a local device?

I'm still a little unclear, but most likely:

  qemu-nbd -c /dev/nbd0 file.qcow2
  qemu-nbd -d /dev/nbd0

The first command will attach file.qcow2 to /dev/nbd0, making it
appear as a normal host device.  The second will detach it.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://people.redhat.com/~rjones/virt-top