Hello,
----- Original Message -----
From: "Adam Litke" agl@us.ibm.com To: "Alon Bar-Lev" alonbl@redhat.com Cc: "Livnat Peer" lpeer@redhat.com, "VDSM Project Development" vdsm-devel@lists.fedorahosted.org Sent: Tuesday, November 27, 2012 12:51:36 AM Subject: Re: [vdsm] Future of Vdsm network configuration - Thread mid-summary
Nice writeup! I like where this is going but see my comments inline below.
On Mon, Nov 26, 2012 at 03:18:22PM -0500, Alon Bar-Lev wrote:
----- Original Message -----
From: "Livnat Peer" lpeer@redhat.com To: "Shu Ming" shuming@linux.vnet.ibm.com Cc: "Alon Bar-Lev" abarlev@redhat.com, "VDSM Project Development" vdsm-devel@lists.fedorahosted.org Sent: Monday, November 26, 2012 2:57:19 PM Subject: Re: [vdsm] Future of Vdsm network configuration - Thread mid-summary
On 26/11/12 03:15, Shu Ming wrote:
Livnat,
Thanks for your summary. I got comments below.
2012-11-25 18:53, Livnat Peer:
Hi All, We have been discussing $subject for a while and I'd like to summarized what we agreed and disagreed on thus far.
The way I see it there are two related discussions:
- Getting VDSM networking stack to be distribution agnostic.
- We are all in agreement that VDSM API should be generic
enough to incorporate multiple implementation. (discussed on this thread: Alon's suggestion, Mark's patch for adding support for netcf etc.)
- We would like to maintain at least one implementation as the
working/up-to-date implementation for our users, this implementation should be distribution agnostic (as we all acknowledge this is an important goal for VDSM). I also think that with the agreement of this community we can choose to change our focus, from time to time, from one implementation to another as we see fit (today it can be OVS+netcf and in a few months we'll use the quantum based implementation if we agree it is better)
- The second discussion is about persisting the network
configuration on the host vs. dynamically retrieving it from a centralized location like the engine. Danken raised a concern that even if going with the dynamic approach the host should persist the management network configuration.
About dynamical retrieving from a centralized location, when will the retrieving start? Just in the very early stage of host booting before network functions? Or after the host startup and in the normal running state of the host? Before retrieving the configuration, how does the host network connecting to the engine? I think we need a basic well known network between hosts and the engine first. Then after the retrieving, hosts should reconfigure the network for later management. However, the timing to retrieve and reconfigure are challenging.
We did not discuss the dynamic approach in details on the list so far and I think this is a good opportunity to start this discussion...
From what was discussed previously I can say that the need for a well known network was raised by danken, it was referred to as the management network, this network would be used for pulling the full host network configuration from the centralized location, at this point the engine.
About the timing for retrieving the configuration, there are several approaches. One of them was described by Alon, and I think he'll join this discussion and maybe put it in his own words, but the idea was to 'keep' the network synchronized at all times. When the host have communication channel to the engine and the engine detects there is a mismatch in the host configuration, the engine initiates 'apply network configuration' action on the host.
Using this approach we'll have a single path of code to maintain and that would reduce code complexity and bugs - That's quoting Alon Bar Lev (Alon I hope I did not twisted your words/idea).
On the other hand the above approach makes local tweaks on the host (done manually by the administrator) much harder.
Any other approaches ?
I'd like to add a more general question to the discussion what are the advantages of taking the dynamic approach? So far I collected two reasons:
-It is a 'cleaner' design, removes complexity on VDSM code, easier to maintain going forward, and less bug prone (I agree with that one, as long as we keep the retrieving configuration mechanism/algorithm simple).
-It adheres to the idea of having a stateless hypervisor - some more input on this point would be appreciated
Any other advantages?
discussing the benefits of having the persisted
Livnat
Sorry for the delay. Some more expansion.
ASSUMPTION
After boot a host running vdsm is able to receive communication from engine. This means that host has legitimate layer 2 configuration and layer 3 configuration for the interface used to communicate to engine.
MISSION
Reduce complexity of implementation, so that only one algorithm is used in order to reach to operative state as far as networking is concerned.
(Storage is extremely similar I can s/network/storage/ and still be relevant).
DESIGN FOCAL POINT
Host running vdsm is a complete slave of its master, will it be ovirt-engine or other engine.
I do not agree with this direction. It reinforces the single point of failure of the centralized manager. Also, I am actively working to make vdsm a self contained component that is independently useful. This proposal will effectively cripple that effort.
I would prefer a statement that a node _CAN_ be a slave to engine but can also re-apply a previous configuration in the absense of a management server. See my other post for how this can be achieved without adding much complexity to the design.
I strongly disagree. I think you going to mix two separate components into one. This is a fundamental issue, so I won't answer all the point you raise because all derives from this one.
vdsm is a slave, and should move to a stateless slave in order to keep it simple and stupid, which is actually smart. A management component can be installed on the same host or at different host. This management component can be a) a custom management component that is not part of the ovirt architecture. b) a component that manages a cluster on behalf of the ovirt-engine.
There is no problem in implementing this component at: a) vdsm protocol proxy, a component that sits between vdsm and the ovirt-engine or whatever north connection. b) entirely different entity which communicate with vdsm and different protocol to north.
If we follow this design, we have simple building blocks, that together can build complex solutions. As each building block is simple the cost of maintenance of each is lower.
Alon.
Having a complete slave ease implementation:
- Master always apply the setting as-is.
- No need to consider slave state.
- No need to implement AI to reach from unknown state X to known
state Y + delta.
These would be properties of any intelligent design regardless if engine is responsible for triggering the configuration changes or if vdsm does it autonomously. In either case you need to write an algorithm that is capable of deleting all networking config (except for the management interface). Without this, you would be unable to apply incremental configuration changes from engine reliably.
- After reboot (or fence) host is always in known state.
Once you have a method to strip networking config down to only the management interface, you can always get back to a known state. I suggest having a vdsm API that can do this for you.
ALGORITHM
A. Given communication to vdsm, construct required vlan, bonding, bridge setup on machine.
B. Reboot/Fence - host is reset, apply A.
C. Network configuration is changed at engine: (1) Drop all resources that are not used by active VMs.
This is the 'network reset' operation I am referring to.
(2) Apply A.
D. Host in maintenance - network configuration can be changed, will be applied when host go into active, apply C (no resources are used by VMs, all resources are dropped).
E. Critical network is down (Host not operative) - network configuration is not changed.
F. Host unreachable (None responsive) - network configuration cannot be changed.
BENEFITS
Single deterministic algorithm to apply network configuration.
Pre-defined state after host reboot/fence, host always reachable, previous network configuration that may be malformed is not in effect.
Do you plan to keep the transactional nature of the current API (ie. setSafeNetworkingConfig must be called after setupNetworks in order to persist it)?
Easy to integrate with various network management solution, can it be primitive iproute, brctl implementation, NetworkManager, OVS or any other configuration, as Linux is Linux is Linux, the ability to interact with the kernel is single, while in order to persist implementation requires to interact with the distribution.
Moreover, a stateless implementation may be integrated with larger set of network management tools, as no assumption of persistence is added to the requirements, so if OVS is non-persistent, we use it as-is.
We should aspire to reach to a state in which ovirt-node or any other similar solution is totally stateless, adding a new node to a cluster should be some blade rebooting from PXE, each persistence layer we drop, the closer we reach to managing a large data center built on huge number of machines go up/down as required joining different clusters.
While discussing clusters, we should also consider autonomic clusters that enforces policy even if ovirt-engine is unreachable, in this mode we would like a primitive manager to be able to enforce policy including networking, while allowing adding/removing nodes without performing any local configuration.
This could also be done without requiring another redundant management entity by storing a fallback config to apply when engine is unreachable. Yes, it's stateful, but that's not always a problem.
IMPLICATIONS
System administrator will not be allowed to modify 'by hand' any of the network settings (except of this basic engine reachability).
Good luck making that requirement stick in the face of real customers :) You'll need (at the very least) a hooking mechanism for admins to override some configuration that hasn't yet been modeled by oVirt.
Special settings can be set in the master, which will apply them via the master->vdsm protocol, which in turn use the network management interface in order to push them, this method should be generic enough to allow pushing most of the configuration setting allowed (key=value). This approach will also help replacing/adding nodes in cluster and/or mass deployment.
Edge conditions can be handled by executing some script on host machine, allowing administrator to override network configuration upon network configuration event.
SUMMARY
Assuming the host running vdsm as a complete slave and stateless will enable us to provide better control over that host in the short and long run.
Manual intervention on hosts serving as hypervisors has the flexibility argument. However at mass deployment, large data-center or dynamic environment this flexibility argument becomes liability.
Today oVirt plays in the small data center realm so I do think it's important to give appropriate weight to the flexibility argument. It should be possible to build different environments based on the needs of the deployment.
-- Adam Litke agl@us.ibm.com IBM Linux Technology Center