Re: [Qemu-devel] Loadable block drivers?

[Lindsay Mathieson]

On 4 April 2018 at 23:41, Stefan Hajnoczi <address@hidden> wrote:

> On Tue, Apr 03, 2018 at 11:30:33AM +0800, Fam Zheng wrote:
> > On Tue, 04/03 13:17, Lindsay Mathieson wrote:
> > > On 3 April 2018 at 13:11, Fam Zheng <address@hidden> wrote:
> > >
> > > > On Tue, 04/03 12:59, Lindsay Mathieson wrote:
> > > > > Hi all, was looking at developing a block driver for qemu - have
> examined
> > > > > the drivers at:
> > > > >
> > > > >   https://github.com/qemu/qemu/tree/master/block
> > > > >
> > > > > And it seems straightforward enough.
> > > > >
> > > > > One thing that is unclear - all the drivers appear to be compiled
> > > > directly
> > > > > into qemu. Is there no way to load them dynamically as .so modules?
> > > >
> > > > './configure --enable-modules' will enable building block drivers as
> .so
> > > > objects, and they are loaded dynamically. These are in-tree .so
> modules;
> > > > out-of-tree modules like in Linux kernel are intentionally forbidden.
> > > >
> > > > Fam
> > > >
> > >
> > >
> > >
> > > Rats, I take it that means I can't develop a testing block module and
> load
> > > it with an pre-existing qemu install.
> >
> > No, that's not possible.
>
> Depending on what you are trying to do, you could use the blkdebug,
> null-co, NBD, or iSCSI drivers to perform your testing.
>
> blkdebug does fault injection (e.g. you can test what happens when
> certain I/O requests fail).
>
> null-co is a nop block driver useful for some types of performance
> testing and it also supports introducing an artificial delays.
>
> NBD and iSCSI can be used to forward I/O requests to an external server
> where you can implement any behavior you want.
>
> We can discuss it more if you can explain what you're trying to do.
>
> Stefan
>



Thanks Stefan, looking to develop a lizardfs block driver. A process that
only involved building a module rather than the entire qemu tree would mike
life easier, especially if I could test it on a live system (proxmox
cluster). A custom qemu install is not an option for that.

-- 
Lindsay