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

[Avi Kivity <avi@redhat.com>]

On 12/05/2009 07:47 PM, Anthony Liguori wrote:
> Avi Kivity wrote:
>> On 12/05/2009 12:29 AM, Anthony Liguori wrote:
>>>
>>> Well disk sharing isn't actually bad as long as it's raw or a 
>>> physical device.
>>>
>>> For qcow2, it's very complicated by backing files because we really 
>>> need to express the concept of a read-write lock.
>>>
>>> That is, as long as the guests are opening the file read only, you 
>>> should allow many readers.  However, you should prevent anyone from 
>>> opening with write permission.  Likewise, writes should prevent 
>>> future reads.
>>>
>>
>> Well shared/exclusive or read/write locks support exactly that.
>>
>> For non-raw storage we should take the locks unconditionally, since 
>> there is no useful way to share such images opened for write access.
>>
>
> I think I made my point poorly.  Consider the following:
>
> qemu-img create -f raw base.img 10G
> qemu-img create -f qcow2 -b base.img cow1.img
> qemu-img create -f qcow2 -b base.img cow2.img
>
> qemu -drive file=cow1.img,lock=exclusive
> qemu -drive file=cow2.img,lock=exclusive
>
> With the current patch, the second command will fail and it's 
> impossible to invoke correctly.  That's because flags are passed down 
> to backing devices directly.  You really need to be much smarter here 
> in how you handle locking.

Oh, that's just an implementation bug.  Obviously there's no need to 
open a file for exclusive access if it will only be accessed for read 
(when merging a snapshot we'd need to upgrade the lock to exclusive 
while that takes place).  I thought you objected to the whole concept.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.