Re: [Libguestfs] Extracting files from OVA is bad
On Fri, Sep 09, 2016 at 01:03:49PM +0200, Tomáš Golembiovský wrote:
Hi,
recently we (oVirt) have started discussing whether the way virt-v2v
handles import from OVA files is good. And I would be interested in
ideas how it can be improved. It is likely somebody already gave some
thought to this problem.
TL;DR: Extracting the OVA before import is a problem for large VMs (in
sizes of TBs). Can we change something to prevent the extraction and
work directly over OVA?
Specifically virt-v2v needs to do:
qemu-img create -b <source-file-within-the-tarball> -f qcow2 overlay.qcow2
qemu-img convert overlay.qcow2 output
What we consider a huge shortcoming is the fact that whole OVA is
extracted prior to the import into a temporary directory and processed
afterwards. Under normal situation user can have up to three copies of
the VM on his drive at the end of import:
* original OVA,
* temporary extracted files (will be deleted when virt-v2v terminates,
* converted VM.
This is not a good idea for large VMs that have hunderds of GBs or even
TBs in size. The requirements on the necessary storage space can be
lessened with proper partitioning. I.e. source OVA and converted VM
don't end up on the same drive and TMPDIR is set to put even temporary
files somewhere else. But this is not a general solution. And sometimes
the necessary space may not be available at all.
The question is how to change the import path so that virt-v2v doesn't
have to extract the OVA. I can see the following solutions:
1) Solve it virt-v2v: create a layer for directly accessing the files
in the archive.
2) Solve it in QEMU: create backing method that would allow creating
qemu disk backed by the archive.
As long as the tar file is not compressed, accessing a file within it
should be trivial.
I asked Kevin if there is a way to get qemu to access a disk image at
an offset within another file, but there is no such feature at the
moment. It's possible with `losetup', but that requires root :-(
(At this point I would normally grumble about how easy this would be
with a microkernel, but I won't do that now.)
David Gilbert suggested looking at qemu-nbd which has an --offset
option, allowing a particular offset with another file to be accessed.
If we wanted to do it entirely within virt-v2v, I think this would be
the way to go - the complex logic could be hidden inside v2v/input_ova.ml
The second problem is to work out the right offset to use. I suspect
this is something that http://www.libarchive.org/ can do, and that
package is also in RHEL.
We could even imagine a qemu block backend based on libarchive.
3) Solve it on oVirt side: use some FUSE-based tool to provide
access to the archive and pass the OVA to virt-v2v not as a file but
as directory.
http://www.cybernoia.de/software/archivemount/ is one such tool which
can do this. It's not in RHEL, but it seems to be based on
libarchive.
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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