Re: [Qemu-devel] [PATCH 1/5] block: added lock image option and callback

[Fam Zheng]

On Tue, 01/12 18:59, Denis V. Lunev wrote:
> On 01/12/2016 02:33 PM, Fam Zheng wrote:
> >On Tue, 01/12 11:10, Kevin Wolf wrote:
> >>The problem is that libvirt already takes a lock, as Dan mentioned in
> >>another reply in this thread, so we can't enable locking in qemu by
> >>default. It would always fail when run under libvirt.
> >>
> >>Unless I'm seriously mistaken, this means that flock() inside qemu is
> >>dead.
> >Yes, I see the problem with libvirt, but can we instead do these?
> >
> >   1) Do a soft flock() in QEMU invocation. If it fails, sliently ignore.
> >   2) Do a hard flock() in qemu-img invocation. If it fails, report and exit.
> >
> >This way, if libvirt is holding flock, we can assume libvirt is actually
> >"using" the image: 1) just works as before, but 2) will not break the qcow2.
> >That is still a slight improvement, and does solve the reckless "qemu-img
> >snapshot create" user's problem.
> >
> >Fam
> There is a better way though.
> 
> If we will switch default in my patch from 'nolock' to 'lock' then
> pour guys which are calling qemu-img etc stuff will see the lock
> as necessary while 'proper management software' aka libvirt
> will be able to call qemu/qemu-img etc with proper 'nolock'
> flag as they do care about the locking.

That is wrong because then we break old libvirt with the new qemu-img (acquires
lock by default), which is IMO a breakage of backward compatibility.

Fam

> 
> Though from my POW all locks should be taken in the responsible
> entity, i.e. qemu or qemu-img as if locks are held by libvirt then
> they should be re-taken on f.e. daemon restart, which could happen.
> This is not that convenient.
> 
> Den