Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Richard W.M. Jones]

On Fri, Dec 04, 2009 at 11:15:12AM -0600, Anthony Liguori wrote:
> Richard W.M. Jones wrote:
>> [from the commit message ...]
>>
>> Allow qemu to acquire shared and exclusive locks on disk images.
>> This is done by extending the -drive option with an additional,
>> optional parameter:
>>
>>   -drive [...],lock=none
>>   -drive [...],lock=shared
>>   -drive [...],lock=exclusive
>>
>> lock=none is the default, and it means that we don't try to acquire
>> any sort of lock.
>>
>> lock=shared tries to acquire a shared lock on the disk image.
>> Multiple instances of qemu may all hold this sort of lock.
>>
>> lock=exclusive tries to acquire an exclusive lock on the disk
>> image.  An exclusive lock excludes all other shared and exclusive
>> locks.
>>
>> If acquisition of a lock fails, opening the image fails.
>>
>> The implementation of locks only works for raw POSIX and Win32
>> files.  However many of the other block types are implemented
>> in terms of these drivers, so they "inherit" locking too.  Other
>> drivers are read-only, so don't require locking.  Below we note
>> only the cases where locking is *not* implemented:
>>
>>   cloop - directly open()s the file, no locking implemented
>>   cow - same as cloop
>>   curl - protocol probably doesn't support locking
>>   nbd - same as curl
>>   
>
> The problem with something like this is that it gives a false sense of  
> security.
>
> You can still run into problems if you have an application accessing the  
> image that doesn't attempt to acquire the lock.

The idea would be for management tools (eg. libvirt) to add the lock
parameter for all virtual machines that they manage.

I don't see how this is worse than what we have now -- ie. no
possibility of locking at all and a very real risk of disk corruption.

Anyway, we could make the locking mandatory.  I believe that the
locking under Win32 (using LockFileEx) is already mandatory.  With
mandatory locks it would be pretty watertight in the single node case.
Alternatively how about making lock=exclusive the default?

Rich.

-- 
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