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From Ronald van Zantvoort <>
Subject Re: [DISCUSS] KVM HA with IPMI Fencing
Date Mon, 19 Oct 2015 09:18:29 GMT
On 16/10/15 00:21, ilya wrote:
> I noticed several attempts to address the issue with KVM HA in Jira and
> Dev ML. As we all know, there are many ways to solve the same problem,
> on our side, we've given it some thought as well - and its on our to do
> list.
> Specifically a mail thread "KVM HA is broken, let's fix it"
> We propose the following solution that in our understanding should cover
> all use cases and provide a fencing mechanism.
> NOTE: Proposed IPMI fencing, is just a script. If you are using HP
> hardware with ILO, it could be an ILO executable with specific
> parameters. In theory - this can be *any* action script not just IPMI.
> Please take few minutes to read this through, to avoid duplicate efforts...
> Proposed FS below:
> ----------------

Hi Ilja, thanks for the design; I've put a comment int 8943, here it is 
verbatim as my 5c in the discussion:

ilya musayev: Thanks for the design document. I can't comment in 
Confluence, so here goes:
When to fence; Simon Weller: Of course you're right that it should be 
highly unlikely that your storage completely dissappears from the 
cluster. Be that as it may, as you yourself note, first of all if you're 
using NFS without HA that likelihood increases manyfold. Secondly, 
defining it as an anlikely disastrous event seems no reason not to take 
it into account; making it a catastrophic event by 'fencing' all 
affected hypervisors will not serve anyone as it would be unexpected and 
The entire concept of fencing exists to absolutely ensure state. 
Specifically in this regard the state of the block devices and their 
data. Marcus Sorensen: For that same reason it's not reasonable to 'just 
assume' VM's gone. There's a ton of failure domains that could cause an 
agent to disconnect from the manager but still have the same VM's 
running, and there's nothing stopping CloudStack from starting the same 
VM twice on the same block devices, with desastrous results. That's why 
you need to know the VM's are very definitely not running anymore, which 
is exactly what fencing is supposed to do.
For this, IPMI fencing is a nice and very often used option; absolutely 
ensuring a hypervisor has died, and ergo the running VM's. It will 
however not fix the case of the mass rebooting hypervisors (but rather 
quite likely making it even more of an adventure if not addressed properly)
Now, with all that in mind, I'd like to make the following comments 
regarding ilya musayev 's design.
First of the IPMI implementation: There's is IMHO no need to define IPMI 
(Executable,Start,Stop,Reboot,Blink,Test). IPMI is a protocol, all these 
are standard commands. For example, using the venerable `ipmitool` gives 
you `chassis power (on,status,poweroff,identify,reset)` etc. which will 
work on any IPMI device; only authentication details (User, Pass, Proto) 
differ. There's bound to be some library that does it without having to 
resort to (possibly numerous) different (versions of) external binaries.
Secondly you're assuming that hypervisors can access the IPMI's of their 
cluster/pod peers; although I'm not against this assumption per sé, I'm 
also not convinced we're servicing everybody by forcing that assumption 
to be true; some kind of IPMI agent/proxy comes to mind, or even 
relegating the task back to the manager or some SystemVM. Also bear in 
mind that you need access to those IPMI's to ensure cluster 
functionality, so a failure domain should be in maintenance state if any 
of the fence devices can't be reached
Thirdly your proposed testing algorithm needs more discussion; after 
all, it directly hits the fundamental principal reasons for why to fence 
a host, and that's a lot more than just 'these disks still gets writes'. 
In fact, by the time you're checking this, you're probably already 
assuming something's very wrong with the hypervisor, so why not just 
fence it then? The decision to fence should lie with the first 
notification that some is (very) wrong with the hypervisor, and only 
limited attempts should be made to get it out. Say it can't reach it's 
storage and that get's you your HA actions; why check for the disks 
first? Try to get the storage back up like 3 times, or for 90 sec or so, 
then fence the fucker and HA the VM's immediately after confirmation. In 
fact, that's exactly what it's doing now, with the side note that 
confirmation can only reasonably follow after the hypervisor is done 
Finally as mentioned you're not solving the 'o look, my storage is gone, 
let's fence' * (N) problem; in the case of a failing NFS:
Every host will start IPMI resetting every other hypervisor; by then 
there's a good chance every hypervisor in all connected clusters are 
rebooting, leaving a state where there's no hypervisors in the cluster 
to fence others; that in turn should lead to the cluster falling in 
maintenance state, which will lead to even more bells & whistles going off.
They'll come back, find the NFS still gone, and continue resetting each 
other like there's no tomorrow
Support staff already panicking over the NFS/network outage now has to 
deal with entire clusters of hypervisors in perpetual reboot as well as 
clusters which are completely unreachable because there's no one left to 
check state; this all while the outage might simply require the revert 
of some inadvertent network ACL snafu
Although I well understand Simon Weller's concerns regarding agent 
complexity in this regard, quorum is the standard way of solving that 
problem. On the other hand, once the Agents start talking to each other 
and the Manager over some standard messaging API/bus this problem might 
well be solved for you; getting, say, Gossip or Paxos or any other 
clustering/quorum protocol shouldn't be that hard considering the amount 
of Java software already doing just that out there.
Another idea would be to introduce some other kind of storage 
monitoring, for example by a SystemVM or something.
If you'll insist on the 'clusters fence themselves' paradigm, you could 
maybe also introduce a constraint that a node is only allowed to fence 
others if itself is healthy; ergo if it doesn't have all storages 
available, it doesn't get to fence others whose storage isn't available.

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