Libguestfs - Oct 2018 - Re: Question about mounting QCOW2 files....

[Adding libguestfs mailing list]

On Sun, Oct 14, 2018 at 11:53:17AM -0400, Raghuram Devarakonda
wrote:
> Hi,
> 
> My name is Raghuram Devarakonda and I am a big fan of "nbdkit". I
have
> successfully used it to prototype (using Python plugin) a complex
> project in our company and since then, I have also been trying to
> understand and learn "guestfs-tools" as well. I have a question
in
> this regard and I hope you don't mind my sending mail gratuitously.
> 
> 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.

Rich.
> Can you please let me know if this is possible? Can I mount a sparse
> file using guestfs-tools even though it is not really a disk?
> 
> Thanks in advance,
> Raghu
-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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On Mon, Oct 15, 2018 at 3:54 AM Richard W.M. Jones <rjones@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?

Thanks,
Raghu

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@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
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