On Tue, 01/12 13:24, Daniel P. Berrange wrote: > On Tue, Jan 12, 2016 at 09:17:51PM +0800, Fam Zheng wrote: > > On Tue, 01/12 13:28, Kevin Wolf wrote: > > > Am 12.01.2016 um 12:33 hat Fam Zheng geschrieben: > > > > 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. > > > > > > This makes two assumptions: > > > > > > 1. qemu is only ever invoked by libvirt > > > 2. qemu-img is only ever invoked by human users > > > > > > Both of them are wrong. 1. just means that manually started QEMUs are > > > unprotected (which is already bad), but 2. means that qemu-img called by > > > libvirt fails (which is obviously not acceptable). > > > > No, my assumptions are: > > > > a. libvirt calls flock() when invoking qemu; > > b. libvirt doesn't call flock() when invoking qemu-img; (if I read the > > libvirt > > code correctly, input from libvirt folks needed); > > b. is /currently/ true, but I wouldn't guarantee that will always be > true, because we've (vague) plans to extend our locking infrastructure > to cover our storage pools APIs too, at which point we'd likely be > have locking around qemu-img based API calls too. There's also likelihood > we'll make our locking API public, in which case it is possible that > an app using libvirt could have acquired locks on the file. >
This is not a problem. When you extend that in libvirt, you can at meanwhile modify it to always specify "nolock=on" when invoking the new qemu-img so that it doesn't check flock(). Fam
