[libvirt] [PATCHv2] Don't allow two or more disks to be mapped to the same image file

[Hu Tao]

On Thu, Mar 24, 2011 at 03:45:26PM -0600, Eric Blake wrote:
> On 03/24/2011 02:46 AM, Hu Tao wrote:
> > If two or more disks are mapped to the same image file, operating
> > on these disks at the same time may corrupt data stored in the
> > image file.
> > 
> > changes:
> > 
> > v2:
> > 
> > - allow it for read-only disks
> > - compare source files by inode number
> > 
> > +
> > +    if (stat(disk->src, &stat1)) {
> > +        if (errno != ENOENT) {
> > +            /* Can't stat file, for safety treate it as conflicted */
> 
> s/treate/treat/
> 
> Won't this will fail on root-squash NFS from qemu:///system?  (Or does
> root-squash meant that root can still stat() but just not open() a file?)

I think it won't. man stat says:

  No permissions are required on the file itself (to stat it)

But needs 'r' bit to open().

> 
> Overall, I'm worried that this patch is repeating some of danpb's bigger
> efforts to integrate a sanlock disk contention avoidance [1].  If a
> resource manager is properly hooked to all disks, then we can prevent
> contention between domains (and not limit ourself to just single-domain
> contention, as in this patch).  On the other hand, this seems like an
> easy enough check to do for a single domain whether or not we get the
> sanlock code completed (that is, timing wise this looks like it could be
> ready prior to 0.9.0 while Dan's work is bigger in scope and probably
> missed the feature freeze for this month's release).  So I'm not sure
> whether to ack this.

Once danpb's sanlock patch is ready we can easily migrate from the
method of this patch to the sanlock. But what is more important is
to protect users' data. Although the chance is rare that a user
attaches a disk using an image file that is already in use, but if
it happens and important data is lost, it will become a disaster
(especially for a productive environment)

> 
> [1] https://www.redhat.com/archives/libvir-list/2011-January/msg00963.html
> 
> -- 
> Eric Blake   eblake at redhat.com    +1-801-349-2682
> Libvirt virtualization library http://libvirt.org
>