[libvirt-users] Snapshots vs filesystem shares

[anonym]

Gentle ping, ~6 weeks later. :)

On 05/16/2015 08:08 PM, anonym wrote:
> [Please keep me Cc:ed since I'm not subscribed to the list!]
> 
> Hi list,
> 
> The tl;dr; is: with Libvirt/QEMU, should it be possible to mount a
> filesystem share (of type='mount') after restoring from a snapshot that
> was taken when the filesystem share wasn't mounted?
> 
> It's pretty clear that Libvirt/QEMU (currently, at least) doesn't
> support taking snapshots of a live guest which has active filesystem
> shares (type='mount'). I get this error:
> 
>     Call to virDomainSaveFlags failed: internal error: unable to execute
>     QEMU command 'migrate': Migration is disabled when VirtFS export
>     path '${TARGET_PATH}' is mounted in the guest using mount_tag
>     '$TAG' (Libvirt::Error)
> 
> I have a use case where I very much would like this combination. It
> wouldn't be a problem if the filesystem shares would have to be
> temporarily unmounted while taking the snapshot, and mounted again after
> restoring it. However, while trying that, `mount` hangs when trying to
> mount a filesystem share again *after* restoring the snapshot
> (unmounting and remounting works perfectly before that, of course).
> Nothing is reported in syslog.
> 
> I've also tried unloading combinations of the various 9p and virtio
> related modules (like 9p, 9pnet_virtio, 9pnet, virtio, etc) before
> taking the snapshot, and then reload them after restoring it, in hope of
> getting them into a sane state again (or whatever is the issue). But
> then I've seen errors like this in syslog:
> 
>     9pnet: Installing 9P2000 support
>     virtio-pci 0000:00:08.0: irq 42 for MSI/MSI-X
>     virtio-pci 0000:00:08.0: irq 43 for MSI/MSI-X
>     virtio-pci 0000:00:08.0: irq 42 for MSI/MSI-X
>     virtio-pci 0000:00:08.0: irq 43 for MSI/MSI-X
>     9pnet_virtio: probe of virtio3 failed with error -2
>     FS-Cache: Loaded
>     9p: Installing v9fs 9p2000 file system support
>     FS-Cache: Netfs '9p' registered for caching
>     9pnet_virtio: no channels available
> 
> and `mount` complains that the source (tag) doesn't exist when trying to
> mount the filesystem share again. For the record, the mount command I
> always use is simply:
> 
>     mount -t 9p -o trans=virtio $TAG $TARGET_DIR
> 
> I've tried setting `-o debug=0xfff` but I get no debug info at all.
> 
> Is it expected behaviour that filesystem shares get into a broken state
> after restoring a snapshot?
> 
> If it's of any relevance, here's some more context:
> 
> * The host is running Debian Jessie with Linux 3.16.7-ckt9-3~deb8u1,
>   Libvirt 1.2.9, QEMU 2.1.
> 
> * The guest is Tails (https://tails.boum.org) which is Debian Wheezy
>   with Linux 3.16.7-ckt9-3.
> 
> I doubt it matters since I tested this ~two years ago, and got (IIRC)
> the exact same results.
> 
> Cheers!


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