Re: [Qemu-devel] Re: [et-mgmt-tools] Image Corruption Possible with qemu and qemu-kvm

[Anthony Liguori <anthony@codemonkey.ws>]

On Mon, 2007-08-27 at 21:27 +0100, Daniel P. Berrange wrote:
> On Mon, Aug 27, 2007 at 10:19:42PM +0200, Sven Oehme wrote:
> > i thought it might be good to post this on qemu-devel as well, as i see 
> > this as a general qemu / kvm / xen /whatevercomesnext issue .
> >
> > are there any plans to implement a default in qemu to prevent accessing 
> > the same image multiple times ? 
> 
> Each QEMU instance is unware of each other so can't directly check this. One
> possiblity would be for QEMU to use fcntl() to take a lock on a disk image
> when opening it. With raw disk images at least, it is, however, safe to let
> multiple QEMU instances use it at once *provided* you have a clustered
> filesystem installed, rather than regular FAT or ext2/3, so one wouldn't want
> to exclude this use case.
> 
> > i know that in xen they deal with this problem somehow xen specific.
> > wouldn't it be good to prevent multiple access to the same image by 
> > default and add a switch to overwrite it ? 
> 
> In the scenario you mention, libvirt should probably do a sanity check for
> this before letting you start the guest. libvirt already supports the idea
> of 'shared' disk images where two or more guests can be optionally configured 
> to have write access - basically assumes the admin requesting sharing knows
> what they're doing.

I think this is the right level myself.  Advisory locks work okay but
not all filesystems support them.  It's particularly nasty when you have
a clustered filesystem in the host.  I think it would do more harm than
good to have a feature like that was supposed to provide a safe-guard
but then frequently didn't work.

Regards,

Anthony Liguori

> > i corrupted a couple of virtual Linux images last week as i accidentally 
> > started them two times .. 
> > i opened a bug against qemu (not sure if this was a good idea) -->  
> > https://bugzilla.redhat.com/show_bug.cgi?id=253533 
> > 
> > are there any plans to ensure that this can't happen in future releases of 
> > virt-manager with lock files in the image directory or similar ? 
> 
> Dan.