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

[Avi Kivity]

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.

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