Re: [Qemu-devel] [PATCH v8 00/36] block: Image locking series

[Fam Zheng <famz@redhat.com>]

On Tue, 10/25 09:06, Richard W.M. Jones wrote:
> On Tue, Oct 25, 2016 at 03:09:51PM +0800, Fam Zheng wrote:
> > On Mon, 10/24 12:11, Kevin Wolf wrote:
> > > Am 22.10.2016 um 03:00 hat Max Reitz geschrieben:
> > > > <parenthesis>
> > > > 
> > > > I personally still don't like making locking a qdev property very much
> > > > because it doesn't make sense to me*. But I remember Kevin had his
> > > > reasons (even though I can no longer remember them) for asking for it,
> > > > and I don't have any besides "It doesn't make sense to me". After having
> > > > though a bit about it (= having written three paragraphs and deleted
> > > > them again), I guess I can make some sense of it, though it seems to be
> > > > a rather esoteric use case still; it appears to me that a guest could
> > > > use it to signal that it's fine with some block device being shared;
> > > > then we could use a shared lock or none at all or I don't know.
> > > > Otherwise, we should get an exclusive lock for write access and a shared
> > > > lock for read access.
> > > 
> > > The reason is pretty simple if you think about this question: Why do we
> > > need user input in the first place? 
> > 
> > I think the reason why we have an option at all is rather because of the special
> > case of libguestfs [1], otherwise locks should just be acquired sensibly as the
> > "auto" mode does.
> 
> It's not just that.
> 
> Qemu doesn't have enough information to make the correct decision
> about locking automatically -- for example, Qemu doesn't know if the
> guest is using cluster filesystems or not (or for some other reason
> the guest wants a shared writable block device, eg. for running Disk
> Paxos).

Yeah, okay, but I agree with Max that this doesn't make any sense on many
emulated device types that inherently aren't sharable in real world (like IDE).
It feels we are trying to solve two problems at the same time which makes a
questionable (qdev) interface.

Fam