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Re: [libvirt] [PATCH] qemu: turn on virtlockd by default

On Thu, Feb 02, 2017 at 12:47:30PM +0000, Richard W.M. Jones wrote:
> On Wed, Feb 01, 2017 at 04:54:01PM +0000, Daniel P. Berrange wrote:
> > The virtlockd daemon has existed for years now, but we have never
> > turned it on by default, requiring explicit user opt-in. This leaves
> > users unprotected against accidents out of the box.
> > 
> > By turning it on by default, users will at least be protected for
> > mistakes involving local files, and files on shared filesystems
> > that support fcntl() (eg NFS).
> What are the implications of this for passively reading
> live disks?  (a la tools such as virt-df)

None - at this time, a disk marked <readonly/> won't acquire any
locks. We map

  <readonly/> -> no lock
 <shareable/> -> fnctl read lock
  default     -> fcntl write lock

In future we'll likely copy the trick qemu has done to use a pair
of fcntl locks on separate bytes, in order to map all combinations
to locks. As long as you always mark disks <readonly/> though you
should be fine no matter what we do.

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