[ClusterLabs] Antw: Re: 2-Node Cluster Pointless?

Tue Apr 18 14:18:42 UTC 2017

On 18/04/17 15:02, Digimer wrote:
> On 18/04/17 10:00 AM, Digimer wrote:
>> On 18/04/17 03:47 AM, Ulrich Windl wrote:
>>>>>> Digimer <lists at alteeve.ca> schrieb am 16.04.2017 um 20:17 in Nachricht
>>> <12cde13f-8bad-a2f1-6834-960ff3afce6c at alteeve.ca>:
>>>> On 16/04/17 01:53 PM, Eric Robinson wrote:
>>>>> I was reading in "Clusters from Scratch" where Beekhof states, "Some would
>>>
>>>> argue that two-node clusters are always pointless, but that is an argument 
>>>> for another time." Is there a page or thread where this argument has been 
>>>> fleshed out? Most of my dozen clusters are 2 nodes. I hate to think they're
>>>
>>>> pointless.  
>>>>>
>>>>> --
>>>>> Eric Robinson
>>>>
>>>> There is a belief that you can't build a reliable cluster without
>>>> quorum. I am of the mind that you *can* build a very reliable 2-node
>>>> cluster. In fact, every cluster our company has deployed, going back
>>>> over five years, has been 2-node and have had exception uptimes.
>>>>
>>>> The confusion comes from the belief that quorum is required and stonith
>>>> is option. The reality is the opposite. I'll come back to this in a minute.
>>>>
>>>> In a two-node cluster, you have two concerns;
>>>>
>>>> 1. If communication between the nodes fail, but both nodes are alive,
>>>> how do you avoid a split brain?
>>>
>>> By killing one of the two parties.
>>>
>>>>
>>>> 2. If you have a two node cluster and enable cluster startup on boot,
>>>> how do you avoid a fence loop?
>>>
>>> I think the problem in the question is using "you" instead of "it" ;-)
>>> Pacemaker assumes all problems that cause STONITH will be solved by STONITH.
>>> That's not always true (e.g. configuration errors). Maybe a node's failcount
>>> should not be reset if the node was fenced.
>>> So you'll avoid a fencing loop, but might end in a state where no resources
>>> are running. IMHO I'd prefer that over a fencing loop.
>>>
>>>>
>>>> Many answer #1 by saying "you need a quorum node to break the tie". In
>>>> some cases, this works, but only when all nodes are behaving in a
>>>> predictable manner.
>>>
>>> All software relies on the fact that it behaves in a predictable manner, BTW.
>>> The problem is not "the predictable manner for all nodes", but the predictable
>>> manner for the cluster.
>>>
>>>>
>>>> Many answer #2 by saying "well, with three nodes, if a node boots and
>>>> can't talk to either other node, it is inquorate and won't do anything".
>>>
>>> "wan't do anything" is also wrong: I must go offline without killing others,
>>> preferrably.
>>>
>>>> This is a valid mechanism, but it is not the only one.
>>>>
>>>> So let me answer these from a 2-node perspective;
>>>>
>>>> 1. You use stonith and the faster node lives, the slower node dies. From
>>>
>>> Isn't there a possibility that both nodes shoot each other? Is there a
>>> guarantee that there will always be one faster node?
>>>
>>>> the moment of comms failure, the cluster blocks (needed with quorum,
>>>> too) and doesn't restore operation until the (slower) peer is in a known
>>>> state; Off. You can bias this by setting a fence delay against your
>>>> preferred node. So say node 1 is the node that normally hosts your
>>>> services, then you add 'delay="15"' to node 1's fence method. This tells
>>>> node 2 to wait 15 seconds before fencing node 1. If both nodes are
>>>> alive, node 2 will be fenced before the timer expires.
>>>
>>> Can only the DC issue fencing?
>>>
>>>>
>>>> 2. In Corosync v2+, there is a 'wait_for_all' option that tells a node
>>>> to not do anything until it is able to talk to the peer node. So in the
>>>> case of a fence after a comms break, the node that reboots will come up,
>>>> fail to reach the survivor node and do nothing more. Perfect.
>>>
>>> Does "do nothing more" mean continuously polling for other nodes?
>>>
>>>>
>>>> Now let me come back to quorum vs. stonith;
>>>>
>>>> Said simply; Quorum is a tool for when everything is working. Fencing is
>>>> a tool for when things go wrong.
>>>
>>> I'd say: Quorum is the tool to decide who'll be alive and who's going to die,
>>> and STONITH is the tool to make nodes die. If everything is working you need
>>> neither quorum nor STONITH.
>>>
>>>>
>>>> Lets assume that your cluster is working find, then for whatever reason,
>>>> node 1 hangs hard. At the time of the freeze, it was hosting a virtual
>>>> IP and an NFS service. Node 2 declares node 1 lost after a period of
>>>> time and decides it needs to take over;
>>>
>>> In case node 1 is DC, isn't a selection for a new DC coming first, and the new
>>> DC doing the STONITH?
>>>
>>>
>>>>
>>>> In the 3-node scenario, without stonith, node 2 reforms a cluster with
>>>> node 3 (quorum node), decides that it is quorate, starts its NFS server
>>>> and takes over the virtual IP. So far, so good... Until node 1 comes out
>>>
>>> Again if node 1 was DC, it's not that simple.
>>>
>>>> of its hang. At that moment, node 1 has no idea time has passed. It has
>>>
>>> You assume no fencing was done...
>>>
>>>> no reason to think "am I still quorate? Are my locks still valid?" It
>>>> just finishes whatever it was in the middle of doing and bam,
>>>> split-brain. At the least, you have two nodes claiming the same IP at
>>>> the same time. At worse, you had uncoordinated writes to shared storage
>>>> and you've corrupted your data.
>>>
>>> But that's no cluster; that's a mess ;-)
>>>
>>>>
>>>> In the 2-node scenario, with stonith, node 2 is always quorate, so after
>>>> declaring node 1 lost, it moves to fence node 1. Once node 1 is fenced,
>>>> *then* it starts NFS, takes over the virtual IP and restores services.
>>>
>>> So you compare "2 nodes + fencing" to "3 nodes without fencing"?
>>>
>>>> In this case, no split-brain is possible because node 1 has rebooted and
>>>> comes up with a fresh state (or it's on fire and never coming back anyway).
>>>>
>>>> This is why quorum is optional and stonith/fencing is not.
>>>
>>> You did not convince me how only one node has the ability to fence the other
>>> without a quorum: Wouldn't both nodes shoot at each other? (I quoted this so
>>> many times, but once again: In HP-UX Service Guard, a lock disk was used as a
>>> tie-breaker: Only one node suceeded to get the lock, and the other committed
>>> suicide (via kernel watchdog timeout)).
>>>
>>>>
>>>> Now, with this said, I won't say that 3+ node clusters are bad. They're
>>>> fine if they suit your use-case, but even with 3+ nodes you still must
>>>> use stonith.
>>>>
>>>> My *personal* arguments in favour of 2-node clusters over 3+ nodes is this;
>>>
>>> Again: You compare "2 nodes with fencing" to "3 nodes without fencing". My
>>> personal vote would be "3 nodes with fencing" if there is enough work for two
>>> nodes.
>>>
>>>>
>>>> A cluster is not beautiful when there is nothing left to add. It is
>>>> beautiful when there is nothing left to take away.
>>>>
>>>> In availability clustering, nothing should ever be more important than
>>>> availability, and availability is a product of simplicity. So in my
>>>> view, a 3-node cluster adds complexity that is avoidable, and so is
>>>> sub-optimal.
>>>
>>> IMHO: a valid cluster software works starting at 1 node, then per induction
>>> also for n+1 nodes. Complexity should grow only linear with the number of
>>> nodes. Of course you shouldn't add nodes just for the number of nodes, but for
>>> the actual need.
>>>
>>> Regards,
>>> Ulrich
>>
>> I was addressing the misconception that fencing was optional and quorum
>> was not. I wrote a longer reply as an article to follow up on this down
>> the thread.
> 
> As an addendum; I will say, with clarity, that *all* clusters need
> stonith, period. 3+ node without stonith is still a disaster waiting to
> happen.
> 

#1 rule of clustering - if we sold clusters in boxes it would be on the
packing tape.

chrissie