Re: [Qemu-devel] n ways block filters

["Benoît Canet" <benoit.canet@irqsave.net>]

The Thursday 20 Mar 2014 à 17:06:26 (+0100), Benoît Canet wrote :
> The Thursday 20 Mar 2014 à 16:12:34 (+0100), Kevin Wolf wrote :
> > 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 <famz@redhat.com> 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.
> 
> Maybe parents BDS could use a generic block function to get a cookie when they
> start to use a children BDS.
> 
> The parent would to
> 
> bs->file_cookie = bdrv_get_cookie(file);
> bs->file = file;
> 
> when choosing to use file as bs file.
> 
> The get cookie method would be
> 
> uint64_t bdrv_get_cookie(bs) {
>    bs->cookie = gen_uuid(bs);
>    return bs->cookie;
> }
> 
> gen_uuid would combine a random 64 bit number with a registry to prevent
> identical cookie generation.
> 
> After this step every BlockDriver method would receive the cookie as second
> parameter.
> 
> For example bdrv_read(bs, cookie, ...)
> 
> So it's easy for a block driver to discriminate based on the cookie and even to
> look up which of his own child is associated to this cookie.
> 
> Best regards
> 
> Benoît
> 
> > 
> > Kevin

Kevin: what do you think of this cookie idea ?
It seems something doable with reasonable small steps.

Best regards

Benoît

> > 
> > > > 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
> > > >