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
