[openstack-dev] [Ironic] Fuel agent proposal
Devananda van der Veen
devananda.vdv at gmail.com
Mon Dec 8 22:23:58 UTC 2014
I'd like to raise this topic for a wider discussion outside of the hallway
track and code reviews, where it has thus far mostly remained.
In previous discussions, my understanding has been that the Fuel team
sought to use Ironic to manage "pets" rather than "cattle" - and doing so
required extending the API and the project's functionality in ways that no
one else on the core team agreed with. Perhaps that understanding was wrong
(or perhaps not), but in any case, there is now a proposal to add a
FuelAgent driver to Ironic. The proposal claims this would meet that teams'
needs without requiring changes to the core of Ironic.
The Problem Description section calls out four things, which have all been
discussed previously (some are here ). I would like to address each one,
invite discussion on whether or not these are, in fact, problems facing
Ironic (not whether they are problems for someone, somewhere), and then ask
why these necessitate a new driver be added to the project.
They are, for reference:
1. limited partition support
2. no software RAID support
3. no LVM support
4. no support for hardware that lacks a BMC
When deploying a partition image (eg, QCOW format), Ironic's PXE deploy
driver performs only the minimal partitioning necessary to fulfill its
mission as an OpenStack service: respect the user's request for root, swap,
and ephemeral partition sizes. When deploying a whole-disk image, Ironic
does not perform any partitioning -- such is left up to the operator who
created the disk image.
Support for arbitrarily complex partition layouts is not required by, nor
does it facilitate, the goal of provisioning physical servers via a common
cloud API. Additionally, as with #3 below, nothing prevents a user from
creating more partitions in unallocated disk space once they have access to
their instance. Therefor, I don't see how Ironic's minimal support for
partitioning is a problem for the project.
There is no support for defining a RAID in Ironic today, at all, whether
software or hardware. Several proposals were floated last cycle; one is
under review right now for DRAC support , and there are multiple call
outs for RAID building in the state machine mega-spec . Any such support
for hardware RAID will necessarily be abstract enough to support multiple
hardware vendor's driver implementations and both in-band creation (via
IPA) and out-of-band creation (via vendor tools).
Given the above, it may become possible to add software RAID support to IPA
in the future, under the same abstraction. This would closely tie the
deploy agent to the images it deploys (the latter image's kernel would be
dependent upon a software RAID built by the former), but this would
necessarily be true for the proposed FuelAgent as well.
I don't see this as a compelling reason to add a new driver to the project.
Instead, we should (plan to) add support for software RAID to the deploy
agent which is already part of the project.
LVM volumes can easily be added by a user (after provisioning) within
unallocated disk space for non-root partitions. I have not yet seen a
compelling argument for doing this within the provisioning phase.
There are already in-tree drivers    which do not require a BMC.
One of these uses SSH to connect and run pre-determined commands. Like the
spec proposal, which states at line 122, "Control via SSH access feature
intended only for experiments in non-production environment," the current
SSHPowerDriver is only meant for testing environments. We could probably
extend this driver to do what the FuelAgent spec proposes, as far as remote
power control for cheap always-on hardware in testing environments with a
(And if anyone wonders about a use case for Ironic without external power
control ... I can only think of one situation where I would rationally ever
want to have a control-plane agent running inside a user-instance: I am
both the operator and the only user of the cloud.)
In summary, as far as I can tell, all of the problem statements upon which
the FuelAgent proposal are based are solvable through incremental changes
in existing drivers, or out of scope for the project entirely. As another
software-based deploy agent, FuelAgent would duplicate the majority of the
functionality which ironic-python-agent has today.
Ironic's driver ecosystem benefits from a diversity of hardware-enablement
drivers. Today, we have two divergent software deployment drivers which
approach image deployment differently: "agent" drivers use a local agent to
prepare a system and download the image; "pxe" drivers use a remote agent
and copy the image over iSCSI. I don't understand how a second driver which
duplicates the functionality we already have, and shares the same goals as
the drivers we already have, is beneficial to the project.
Doing the same thing twice just increases the burden on the team; we're all
working on the same problems, so let's do it together.
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