[Qemu-devel] [PATCH] add watermark reporting for block devices

[Francesco Romani]

Hello everyone

I'm one of the oVirt developers (http://www.ovirt.org);
oVirt is a virtualization management application built
around qemu/kvm, so it is nice to get in touch :)

We have begun a big scalability improvement effort, aiming to
support without problems hundreds of VMs per host, with plans
to support thousands in a not so distant future.
In doing so, we are reviewing our usage flows.

One of them is thin-provisioned storage, which is used
quite extensively, with block devices (ISCSI for example)
and COW images.
When using thin provisioning, oVirt tries hard to hide this
fact from the guest OS, and to do so watches closely
the usage of the device, and resize it when its usage exceeds
a configured threshold (the "high water mark"), in order
to avoid the guest OS to get paused for space exhausted.

To do the watching, we poll he devices using libvirt
(virDomainGetBlockInfo), which in turn uses query-blockstats.
This is suboptimal with just one VM, but with hundereds of them,
let alone thousands, it doesn't scale and it is quite a resource
hog.

Would be great to have this watermark concept supported into qemu,
with a new event to be raised when the limit is crossed.

To track this RFE I filed https://bugs.launchpad.net/qemu/+bug/1338957

Moreover, I had the chance to take a look at the QEMU sources
and come up with this tentative patch which I'd also like
to submit.

Comments and thoughts very welcome!

Thanks and best regards,

Francesco Romani (1):
  block: add watermark event

 block.c                   | 56 +++++++++++++++++++++++++++++++++++++++++++++++
 blockdev.c                | 21 ++++++++++++++++++
 include/block/block.h     |  2 ++
 include/block/block_int.h |  3 +++
 qapi/block-core.json      | 33 ++++++++++++++++++++++++++++
 qmp-commands.hx           | 24 ++++++++++++++++++++
 6 files changed, 139 insertions(+)

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1.9.3