[libvirt] Some problem with the save function

[Daniel P. Berrange]

On Wed, Sep 16, 2009 at 05:05:36PM +0200, Daniel Berteaud wrote:
> Hi everyone. This is my first post to this list, so excuse me if it's
> not the right place to poste info like this.
> 
> I've found some problems in qemu_driver.c in the latest versions of
> libvirt:
> 
> - the first problem was also present in 0.7.0. If we run qemu guests
> unprivileged, we cannot use the save function (virsh save
> guestname /path/to/file). The reason seems to be that libvirt first
> create the destination file to write the header and the XML definition.
> The file is owned by root with 600 permission. Then, we ask qemu to
> append the dump of the memory (via the migrate exec: call) to this file.
> This part doesn't work because the qemu user cannot write to this file.
> So we endup with a file which only contains the header and the XML, and
> the guest is stoped :/

Doh, that's a nice problem. You can work around it by editing
/etc/libvirt/qemu.conf and telling qemu to run as root:root
again.  For a real fix we'll need to make the save method 
run 'fchown' on the file descriptor after writing the header.
And then fchown it back to root:root once complete, to stop
any other guest overwriting it.

> - the second problem is present since libvirt 0.7.1. Now that the saved
> file can be compressed, it seems we cannot save in a raw format any
> more. This is due to this part in the code (qemu_driver.c):
> 
> if (STREQ (prog, "raw"))
>         prog = "cat";
>     internalret = virAsprintf(&command, "migrate \"exec:"
>                               "%s -c >> '%s' 2>/dev/null\"", prog,
> safe_path);
> 
> which result in "migrate \"exec cat -c >> safe_path 2>/dev/null\""
> 
> But cat doesn't support the -c argument, so once again, the save fails,
> as we end up with a save file which only contains the header and the XML
> definition.

Wierd, I don't know where/when we gained a '-c' arg to cat but it
looks rather bogus. 

> - the third problem, is that restore doesn't work any more. I haven't
> dig it yet, but when I try to restore a guest using a saved file
> (uncompressed produced under libvirt 0.7.0), I've this error:
> 
> virsh restore /tmp/guest.state
> error: Failed to restore domain from /tmp/guest.state
> error: internal error unable to start guest: 

Hmm, bad error message :-(  We might also need todo a chown()
in the restore path to allow QEMU to read it. NB there is no
compatability between QEMU version, so if you have upgraded
your install of QEMU between the time you saved & restored
it is very likely to crash & burn.

This is something QEMU devs are working on fixing by allowing
for stable machine ABI (available in Fedora 12), and stable
pci device addressing (targetted for Fedora 13).

Regards,
Daniel
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