[libvirt] [PATCH] blockcopy: check dst = identical device
Chun Yan Liu
cyliu at suse.com
Fri Aug 1 08:06:03 UTC 2014
>>> On 7/31/2014 at 05:52 PM, in message <53DA11DB.FE6 : 102 : 21807>, Chun Yan Liu
wrote:
>
>>>> On 7/31/2014 at 11:35 AM, in message <53D9B993.4040806 at redhat.com>, Eric Blake
> <eblake at redhat.com> 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'.
About the 2nd point, 'lockd' manager cannot handle identical device but
different name (extra / in name or softlink), I didn't think of a quick fix.
Maybe still better to check src and dst name in earlier stage. Could call
realpath() to cover more cases. Any other ideas?
>>
>> > 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
>>
>>
>
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