Re: [libvirt] [PATCH] blockcopy: check dst = identical device

>> On 7/31/2014 at 05:52 PM, in message <53DA11DB.FE6 : 102 : 21807>, Chun Yan Liu

>>> On 7/31/2014 at 11:35 AM, in message <53D9B993.4040806(a)redhat.com&gt;, Eric Blake <eblake(a)redhat.com&gt; wrote: > On 07/30/2014 09:29 PM, Chun Yan Liu wrote: > > >> A better idea would be to rely on the volume lease manager - obtaining a > >> lease should be impossible for an image already in use (and should even > >> cover the case of copying 'base <- active' onto 'base', which your > >> equality test wouldn't catch). I'm not sure why the lease manager is not > >> already flagging this issue - are we still using the nop lease manager > >> by default, and would the fcntl or sanlock lease manager do a better job? > > > > Besides the default lock is 'nop', currently lock manager is only used in: > > VM start/stop and attach/detach disk, blockcopy not using it. > > But that's not true - the code IS trying to use it. qemuDomainBlockCopy > calls qemuDomainPrepareDiskChainElement(), which calls > virDomainLockImageAttach(), and that should be the use of the lock > manager. Can you debug why it is not working when using something other > than the 'nop' manager? Update: 1. identical device with exactly the same spelled name. e.g. source is /dev/sda4, dest is /dev/sda4: * 'lockd' manager is working well with --reuse-external. * my previous testing result which makes the VM cannot be started successfully after doing blockcopy to same device and shutdown VM and start VM again, because I didn't add --reuse-external option. Then in code, it considers the dest as a new created file, when error happens, it unlinks the dest, which s actually also my source disk. 2. identical device with not exactly the same spelling. e.g. source is /dev/sda4, dest is /dev///sda4, or a softlink: 'lockd' manager is not working. Both hashtable check and fcntl can't give right result. * HashTable checks the resource in the locked list depending on the name. If name is not exactly the same, it will treat as 'not found in locked list'. * In fcntl stage, since fcntl is to one process, in the same process, do two times fcntl F_SETLK, both will succeed. Since everytime it's virtlockd tries fcntl, so in this case, 1st time fcntl /dev/sda4, it succeeds; 2nd time fcntl /dev///sda4, also succeeds. Not as we expected 'cannot get the lock'.

> > > To use it in blockcopy, maybe can refer to attach/detach disk: before doing > > blockcopy, try AcquireResource; after blockcopy finish, try > releaseResource. > > That should be what is already happening. > > > -- > Eric Blake eblake redhat com +1-919-301-3266 > Libvirt virtualization library http://libvirt.org > >