[Qemu-devel] nbd-server / monitor to copy files/directory in to running VM.

[Qemu-devel] nbd-server / monitor to copy files/directory in to running VM.

[Jd]

Hi
   I see an option to start nbd-server from running VM via qemu monitor. 
This is very useful. The option seems to allow sharing the disk with 
read/write permission. Can I really export and use the nbd-served disk 
from running vm in read/write ? Isnt there a risk for corrupting the 
file system ?

   qemu monitor seems to have command to open, write, close commands. I 
tried them and I could create a file (after going through encoding). How 
can I create a directory using this ?

   I read little bit about live-guestfs, but it needs another agent in 
to the VM. Also, is this ready for prime time ? Any plans to have this 
in the qemu guest agent. This way we dont have to have multiple agents 
required in the guest.

   Is there any other ideas on copying files in to a running VM ?

Thanks
/Jd

Re: [Qemu-devel] nbd-server / monitor to copy files/directory in to running VM.

[Paolo Bonzini]

On 10/28/2014 06:02 PM, Jd wrote:
> Hi
>   I see an option to start nbd-server from running VM via qemu monitor.
> This is very useful. The option seems to allow sharing the disk with
> read/write permission. Can I really export and use the nbd-served disk
> from running vm in read/write ? Isnt there a risk for corrupting the
> file system ?

You must know what you are doing.  In general the answer is yes, you
risk corrupting the file system.

Paolo

>   qemu monitor seems to have command to open, write, close commands. I
> tried them and I could create a file (after going through encoding). How
> can I create a directory using this ?
> 
>   I read little bit about live-guestfs, but it needs another agent in to
> the VM. Also, is this ready for prime time ? Any plans to have this in
> the qemu guest agent. This way we dont have to have multiple agents
> required in the guest.
> 
>   Is there any other ideas on copying files in to a running VM ?
> 
> Thanks
> /Jd
> 
> 

Re: [Qemu-devel] nbd-server / monitor to copy files/directory in to running VM.

[Stefan Hajnoczi]

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On Tue, Oct 28, 2014 at 10:02:21AM -0700, Jd wrote:
>   qemu monitor seems to have command to open, write, close commands. I tried
> them and I could create a file (after going through encoding). How can I
> create a directory using this ?

Do you mean the code in qga/?  That is the QEMU guest agent.  It does
not have a command to create directories.

>   I read little bit about live-guestfs, but it needs another agent in to the
> VM. Also, is this ready for prime time ? Any plans to have this in the qemu
> guest agent. This way we dont have to have multiple agents required in the
> guest.

live-guestfs?  Never heard of it.

Do you mean libguestfs?  That's a tool for disk image manipulation.
Very powerful but works on offline guest disk images.  It does not work
while the guest is running.

>   Is there any other ideas on copying files in to a running VM ?

The same network transports that you would use on a physical machine:
HTTP, CIFS, NFS.

Or an emulated device like a USB Mass Storage Device (like hot-plugging
a thumbdrive into a computer) or the experimental USB Media Transfer
Protocol emulation.

Stefan

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Re: [Qemu-devel] nbd-server / monitor to copy files/directory in to running VM.

[Daniel P. Berrange]

On Wed, Oct 29, 2014 at 03:20:28PM +0000, Stefan Hajnoczi wrote:
> On Tue, Oct 28, 2014 at 10:02:21AM -0700, Jd wrote:
> >   qemu monitor seems to have command to open, write, close commands. I tried
> > them and I could create a file (after going through encoding). How can I
> > create a directory using this ?
> 
> Do you mean the code in qga/?  That is the QEMU guest agent.  It does
> not have a command to create directories.
> 
> >   I read little bit about live-guestfs, but it needs another agent in to the
> > VM. Also, is this ready for prime time ? Any plans to have this in the qemu
> > guest agent. This way we dont have to have multiple agents required in the
> > guest.
> 
> live-guestfs?  Never heard of it.
> 
> Do you mean libguestfs?  That's a tool for disk image manipulation.
> Very powerful but works on offline guest disk images.  It does not work
> while the guest is running.

Regular libguestsfs will run against an live guest, but it restricts
itself to readonly mode in that case since you can't modify disks behind
the back of the guest OS, or QEMU itself.

libguestfs does have a mode where it *can* run against a live guest. You
need to install the libguestfs daemon in the guest and setup a virtio-serial
channel in your guest so it can safely access the guest that way:

  http://rwmj.wordpress.com/2011/07/06/libguestfs-live/

The QEMU NBD server won't be much help with this though. The NBD server
in QEMU is still running behind the back of the guest OS, so it must
only export the disks in read-only mode.  If the QEMU NDB server were
to export in read-write mode, any writes would corrupt the disks from
the POV of the guest OS.  Also the libguestfs daemon always *wants* to
be inside a guest, because it relies on Linux kernel to access the
various filesystems. So even in read-only mode QMEU NBD server doesn't
really do anything immediately useful for libguestfs vs current state
of its code.

Regards,
Daniel
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