Hi all,

Very shortly Mark will be sending some patches for review that implement the
long-awaited integration of mom with vdsm.  I felt it would be easier to
understand the changes to vdsm if they were explained a bit better.  In support
of this I have created a wiki page on ovirt.org with a diagram:


To facilitate discussion, here is the text of that page:

As discussed at the oVirt Workshop and elsewhere, integrating mom with vdsm will
benefit oVirt by providing a mechanism for dynamic, policy-based tuning.  This
mechanism will pave the way for implementing memory ballooning policies, can
enhance migration policy, and will replace the existing ksm tuning thread.

MOM exists today as an independent library that can be used by python programs
such as vdsm or in standalone mode (by using the accompanying momd program.
Mom's operation is very configurable.  The management policy is written in a
Fortran-like language and is replaceable by the end user.  Additionally, plugins
allow you to customize the types of information collected and the manner in
which it is collected.  Similarly, Controller plugins permit a completely
flexible control API to be created.

To integrate mom, vdsm will initialize the mom library in a new thread and start
it.  Therefore, mom and vdsm will exist in the same process.  Vdsm will
configure the mom instance to use plugins and a policy that exclusively target
the vdsm API.  All statistics collection will occur via API calls and any
management actions (including adjustments to KSM and VM balloons) will be done
through the vdsm api as well.  Mom will not use libvirt at all (not even to
monitor for new VMs on the system).

Packaging logistics:
Mom is an independent package that is already in Fedora.  Any changes to mom
that are required to support this integration will be submitted to the mom
project for inclusion.  Vdsm will consume the standard MOM package as a python

In order to control its mom instance, vdsm will ship a mom configuration file
and a mom policy file that will set mom's default behavior.  At startup, vdsmd
will import mom and initialize it with the configuration and policy files.  From
that point on, mom will interact with vdsm through the well-defined API in

New features needed in vdsm:
In order to fully benefit from mom's capabilities, vdsm should implement the
following extra features/APIs:

- Collection of more memory statistics via ovirt-guest-agent including the
  current memory balloon value.
- A vmBalloon API to set a new balloon target.

Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center

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