[ovirt-devel] "Host.queryVms": A proposal for new a VDSM API verb

[Vinzenz Feenstra]

Hi,

This proposal is a follow up on a previous discussion about
optimizing the communication between VDSM and the ovirt-engine.


Currently the engine polls VDSM every 3 seconds with the `list`
verb to retrieve the status of all VMs. Every 5th time (every 15
seconds) it alternates to 'getAllVmStats' which gives more
information about the VMs.


Quite a big portion of the data, mainly the reply of getAllVmStats,
is only of interest if it actually changes so that the engine/back-end
can act on changes and update rows accordingly.

Now the problem is that there is quite a lot of data transferred
which is not really necessary and there is a 15 seconds delay on
communicating changes to certain VMs before the engine actually
can react on them and before these changes can be communicated
to the user.

As part of an improvement on this matter I'd like to propose a new API
verb for VDSM:

Host.queryVms(vmIds=[], fields=[], exclude=[], changedSince='')

This new verb is intended to eventually replace 2 currently
used verbs `list` and `getAllVmStats`. `queryVms` is supposed to
allow to request any data fields of a VM which can be requested through
the public API:

- Statistics
- Configuration
- Status information
- Guest OS Information

I have executed some tests and in those tested scenarios the new Verb
can result in an improvement of 75%-90% of data transferred and average
response body size depending on the scenario and usage.

The test results can be found here:
http://www.ovirt.org/Feature/VDSM_VM_Query_API/Measurements#Results
(An explanation of the tested methods is on the top of the page and a
description of the scenario in each section)

The benefit of introducing this verb would be a lowered volume of data
transferred over the management network which avoids congestion
on huge setups and can introduce an increased responsiveness for
communicating changes in state to the user. For example live migration
status. However even SLA related changes could be communicated faster
for things like QoS.


Preliminary Feature Proposal Wiki:
http://www.ovirt.org/Feature/VDSM_VM_Query_API

-- 
Regards,

Vinzenz Feenstra | Senior Software Engineer
RedHat Engineering Virtualization R & D
Phone: +420 532 294 625
IRC: vfeenstr or evilissimo

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