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

From: Richard W.M. Jones
Subject: Re: [Qemu-block] [PATCH v8 00/36] block: Image locking series
Date: Tue, 25 Oct 2016 09:06:15 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

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


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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