[ovirt-devel] status update: consuming the BLOCK_THRESHOLD event from libvirt (rhbz#1181665)

[Roy Golan]

On Wed, Jul 19, 2017 at 12:46 PM Francesco Romani <fromani at redhat.com>
wrote:

> Hi all,
>
>
> With libvirt 3.2.0 and onwards, it seems we have now the tools to solve
> https://bugzilla.redhat.com/show_bug.cgi?id=1181665
>
> and eventually get rid of the disk polling we do. This change is
> expected to have huge impact on performance, so I'm working on it.
>
>
> I had plans for a comprehensive refactoring in this area, but looks like
> a solution backportable for 4.1.z is appealing, so I
>
> started with this first, saving the refactoring (which I still very much
> want) for later.
>
>
> So, quick summary: libvirt >= 3.2.0 allows to set a threshold to any
> node in the backing chain of each drive of a VM
>
> (
> https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainSetBlockThreshold
> ),
> and fire one event exactly once
>
> when that threshold is crossed. The event needs to be explicitely
> rearmed after.
>
> This is exactly what we need to get rid of polling in the steady state,
> so far so good.
>
>
> The problem is: we can't use this for some important flows we have, and
> which involve usage of disks not (yet) attached to a given VM.
>
>
>


> Possibly affected flows:
>
> - live storage migration:
>
>   we use         flags = (libvirt.VIR_DOMAIN_BLOCK_COPY_SHALLOW |
>                  libvirt.VIR_DOMAIN_BLOCK_COPY_REUSE_EXT |
>                  VIR_DOMAIN_BLOCK_COPY_TRANSIENT_JOB)
>
>   meaning that Vdsm is in charge of handling the volume
>
> - snapshots:
>
>   we use         snapFlags = (libvirt.VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT
> |
>                      libvirt.VIR_DOMAIN_SNAPSHOT_CREATE_NO_METADATA)
>
>
>   (same meaning as above)
>
> - live merge: should be OK (according to a glance at the source and a
> chat with Adam).
>
>
> So looks like we will need to bridge this gap.
>
>
> So we can still use the BLOCK_THRESHOLD event for steady state, and
> avoid polling in the vast majority of the cases.
>
> With "steady state" I mean that the VM is running, with no
> administration (snapshot, live merge, live storage migration...)
> operation in progress.
>
> I think it is fair to assume that VMs are in this state the vast
> majority of the time.
> For the very important cases on which we cannot depend on events, we can
> fall back to polling, but in a smarter way:
>
> instead of polling everything every 2s, let's just poll just the drives
> involved in the ongoing operations.
>
> Those should be far less of the total amount of drives, and for a far
> shorter time than today, so polling should be practical.
>
> Since the event fires once, we will need to rearm it only if the
> operation is ongoing, and only just before to start it (both conditions
> easy to check)
> We can disable the polling on completion, or on error. This per se is
> easy, but we will need a careful review of the flows, and perhaps some
> safety nets in place.
>
>
Consider fusing polling and events into a single pipeline of events so they
can be used together. If a poll triggers an event (with distinguished
origin)
then it all the handling is done in one place and it should be easy to stop
or start polling, or remove them totally.


> Anyway, should we miss to disable the polling, we will "just" have some
> overhead.
>
>

> On recovery, we will need to make sure to rearm all the relevant events,
> but  we can just plug in the recovery we must do already, so this should
> be easy as well.
>
>
What is needed in order to 'rearm' it? is there an API to get the state of
event subscription?
If we lost an event how do we know to rearm it? is it idempotent to rearm?

Remind me, do we extend a disk if the VM paused with out of space event?

How will we handle 2 subsequent events if we didn't extend between them?
(expecting the extend to be async operation)

So it seems to me this could fly and we can actually have the
> performance benefits of events.
>
>
However, due to the fact that we need to review some existing and
> delicate flows, I think we should still keep the current polling code
> around for the next release.
>
> +1


> I believe the best route is:
>
> 1. offer the new event-based code for 4.2, keep the polling around.
> Default to events for performance
>
> 2. remove the polling completely in 4.3
>
>
> Still wonder if removing them totally is good. The absence of the events
should be supervised somehow - like in today, a failure to poll getstats of
a domain will result in a VM going unresponsive. Not the most accurate
state but at least gives some visibility. So polling should cover us where
events will fail. (similar to engine's vms monitoring)

I'm currently working on the patches here:
>
> https://gerrit.ovirt.org/#/q/status:open+project:vdsm+branch:master+topic:watermark-event-minimal
>
>
> Even though the basics are in place, I don't think they are ready for
> review yet.
>
>
> Comments welcome, as usual.
>
>
> --
> Francesco Romani
> Senior SW Eng., Virtualization R&D
> Red Hat
> IRC: fromani github: @fromanirh
>
> _______________________________________________
> Devel mailing list
> Devel at ovirt.org
> http://lists.ovirt.org/mailman/listinfo/devel
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ovirt.org/pipermail/devel/attachments/20170723/75f15ca0/attachment.html>