Re: [Qemu-devel] n ways block filters

[Kevin Wolf]

Am 20.03.2014 um 15:05 hat Benoît Canet geschrieben:
> The Tuesday 18 Mar 2014 à 14:27:47 (+0100), Kevin Wolf wrote :
> > Am 17.03.2014 um 17:02 hat Stefan Hajnoczi geschrieben:
> > > On Mon, Mar 17, 2014 at 4:12 AM, Fam Zheng <address@hidden> wrote:
> > > > On Fri, 03/14 16:57, Benoît Canet wrote:
> > > >> I discussed a bit with Stefan on the list and we came to the 
> > > >> conclusion that the
> > > >> block filter API need group support.
> > > >>
> > > >> filter group:
> > > >> -------------
> > > >>
> > > >> My current plan to implement this is to add the following fields to 
> > > >> the BlockDriver
> > > >> structure.
> > > >>
> > > >> int bdrv_add_filter_group(const char *name, QDict options);
> > > >> int bdrv_reconfigure_filter_group(const char *name, QDict options);
> > > >> int bdrv_destroy_filter_group(const char *name);
> > 
> > Benoît, your mail left me puzzled. You didn't really describe the
> > problem that you're solving, nor what the QDict options actually
> > contains or what a filter group even is.
> > 
> > > >> These three extra method would allow to create, reconfigure or destroy 
> > > >> a block
> > > >> filter group. A block filter group contain the shared or non shared 
> > > >> state of the
> > > >> blockfilter. For throttling it would contains the ThrottleState 
> > > >> structure.
> > > >>
> > > >> Each block filter driver would contains a linked list of linked list 
> > > >> where the
> > > >> BDS are registered grouped by filter groups state.
> > > >
> > > > Sorry I don't fully understand this. Does a filter group contain 
> > > > multiple block
> > > > filters, and every block filter has effect on multiple BDSes? Could you 
> > > > give an
> > > > example?
> > > 
> > > Just to why a "group" mechanism is useful:
> > > 
> > > You want to impose a 2000 IOPS limit for the entire VM.  Currently
> > > this is not possible because each drive has its own throttling state.
> > > 
> > > We need a way to say certain drives are part of a group.  All drives
> > > in a group share the same throttling state and therefore a 2000 IOPS
> > > limit is shared amongst them.
> > 
> > Now at least I have an idea what you're all talking about, but it's
> > still not obvious to me how the three functions from above solve your
> > problem or how they work in detail.
> > 
> > The obvious solution, using often discussed blockdev-add concepts, is:
> >                  ______________
> > virtio-blk_A --> |            | --> qcow2_A --> raw-posix_A
> >                  | throttling |
> > virtio_blk_B --> |____________| --> qcow2_B --> nbd_B
> 
> My proposal would be:
>                  ______________
> virtio-blk_A --> | BDS 1      | --> qcow2_A --> raw-posix_A
>                  |____________|
>                       |
>                  _____|________
>                  |            |  The shared state is the state of a BDS group
>                  | Shared     |  It's stored in a static linked list of the
>                  | State      |  block/throttle.c module. It has a name and 
> contains a
>                  |____________|  throttle state structure.
>                       |
>                  _____|________
>                  |  BDS 2     |
> virtio_blk_B --> |____________| --> qcow2_B --> nbd_B

Okay. I think your proposal might be easier to implement in the short
run, but it introduces an additional type of nodes to the graph (so far
we have only one type, BlockDriverStates) with their own set of
functions, and I assume monitor commands, for management.

This makes the whole graph less uniform and consistent. There may be
cases where this is necessary or at least tolerable because the fully
generic alternativ isn't doable. I'm not convinced yet that this is the
case here.

In contrast, my approach would require considerable infrastructure work
(you somehow seem to attract that kind of things ;-)), but it's merely a
generalisation of what we already have and as such fits nicely in the
graph.

We already have multiple children of BDS nodes. And we take it for
granted that they don't refer to the same data, but that bs->file and
bs->backing_hd have actually different semantics.

We have recently introduced refcounts for BDSes so that one BDS can now
have multiple parents, too, as a first step towards symmetry. The
logical extension is that these parent get different semantics, just
like the children have different semantics.

Doing the abstraction in one model right instead of adding hacks that
don't really fit in but are easy to implement has paid off in the past.
I'm pretty sure that extending the infrastructure this way will find
more users than just I/O throttling, and that having different parents
in different roles is universally useful. With qcow2 exposing the
snapshots, too, I already named a second potential user of the
infrastructure.

> The name of the shared state is the throttle group name.
> The three added methods are used to add, configure and destroy such shared
> states.
> 
> The benefit of this aproach is that we don't need to add a special slot 
> mechanism
> and that removing BDS 2 would be easy.
> Your approach don't deal with the fact that the throttling group membership 
> can
> be changed dynamically while the vm is running: for example adding qcow2_C and
> removing qcow2_B should be made easy.

Yes, this is right. But then, the nice thing about it is that I stayed
fully within the one uniform graph. We just need a way to modify the
edges in this graph (and we already need that to insert/delete filters)
and you get this special case and many others for free.

So, I vote for investing into a uniform infrastructure here instead of
adding new one-off node types.

Kevin

> > That is, the I/O throttling BDS is referenced by two devices instead of
> > just one and it associates one 'input' with one 'output'. Once we have
> > BlockBackend, we would have two BBs, but still only one throttling
> > BDS.
> > 
> > The new thing that you get there is that the throttling driver has
> > not only multiple parents (that part exists today), but it behaves
> > differently depending on who called it. So we need to provide some way
> > for one BDS to expose multiple slots or whatever you want to call them
> > that users can attach to.
> > 
> > This is, by the way, the very same thing as would be required for
> > exposing qcow2 internal snapshots (read-only) while the VM is running.
> > 
> > Kevin
> >