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New HPE StoreVirtual 2-Node Quorum: The Power of 2

StorageExperts

 

The power of 2 is impressive in athletics but it's even more impactful when it comes to achieving high availability and resolving “split-brain” syndrome. That’s the power behind the HPE StoreVirtual 2-Node Quorum feature.

StoreVirtual 2Node BLOG.jpgAs I watched the Olympics last month, it struck me just how impressive athletes can be when they work in tandem. It’s not the first time I was impressed by the power of 2.

The number 2 seems to be the minimum number required to achieve high availability. It started with 2 hard drives, the minimum number required to implement disk RAID. Then came redundant power supplies and network adapters, and along with server virtualization came the concept of server clusters protecting against an entire server failure. While N+1 is always best practice, a minimum of 2 hosts is required for just about any type of server cluster. This includes software-defined storage clusters.

Resolving “split-brain” syndrome in a 2-node storage cluster

In a true active/active or “synchronous” storage cluster, the data is always up to date on all nodes in the cluster. The problem arises when there are only two hosts and one goes down. The surviving node is uncertain whether the other host is down or if its own network connection has been severed. Some IT administrators I know call this “split brain.” Generally, the surviving node will avoid servicing clients to avoid data corruption.

To avoid split brain syndrome in your data center, a “quorum” service is deployed to act as a tie breaker. The concept is simple. If the active node cannot reach the other, but can reach quorum then it knows it is still online and can continue servicing clients without risk of data corruption. In the past, StoreVirtual required a witness VM to be deployed on a third piece of hardware to provide that functionality. The downside was that businesses with a high number of remote sites required a high number of extra VMs running in the environment.

2-Node Quorum sets up a tie-breaker without additional storage hardware

HPE StoreVirtual storage has resolved the split-brain issues plaguing traditional 2-node deployments by leveraging an NFS export to provide quorum. The new StoreVirtual 2-Node Quorum feature sets up a Quorum Witness on an NFS share to act as the tie-breaker. Organizations can now get high availability via any number of 2-node deployments without the associated cost and complexity. Score 2 for StoreVirtual.

We’ve created a short video to show you how this works.

You can try it out for yourself with our free 1TB StoreVirtual VSA software. Install 1TB VSA on 2 virtualized servers and follow the steps outlined in the video to set up 2-Node Quorum. Your 3-year license is automatically activated when you install VSA.

Rich Holmes.jpg

Meet Around the Storage Block Blogger Rich Holmes, HPE Software-Defined Storage Specialist.

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Comments
manuelMA

hi, this server (quourom) you say that can be vm o physical, it can be a vm in the same cluster where the strorevirtual vsa live o must have to install it in a separate infraestructure?

HPEStorageGuy

@ManuelMA - Here's a link to an article talking about using the Quorum Witness. The other option is what has been available for some time which is the "Failover Manager" or FOM.  It's a VM.  Here's a post from a Patrick Terlisten, blogger that uses StoreVirtual and also explains things. 

wallewek

I must not understand this 2-node quorum functionality correctly because, if I do, it still has a weakness.   

As I understand it, the quorum witness is provided by a separate (NFSv3) file share provided by the two nodes of the HA cluster, albeit not a part of the primary HA iSCSI cluster,

If the NFSv3 witness share is highly available, it implies that, in the event of network failure, both hosts could continue to independently host the NFSv3 share, so both could think they have quorum, continuing to run in split-brain mode. 

If the NFSv3 quorum witness share is NOT running as an HA service, that would mean that there is the potential for a host failure that would also stop the witness, which would prevent either host from having quorum.

It is therefore unclear to me how the use of a separate NFSv3 file service, still running on the same two hosts, prevents either split-brain mode or full cluster failure. 

I would really be interested in a clarification of that point,

HPEStorageGuy

@wallewek - apologies as I've been traveling internationally and not able to respond to questions. I would suggest asking your question on the StoreVirtual Forum on our HPE Community website. I'll see if I can get an answer but you'll probably get faster responses by posting technical questions in the forum. 

HPEStorageGuy

@wallewek - got a reply from on of field guys. Here's what he said:

The NFSv3 Witness Share and the Fail Over Manager (FOM) are both run outside the other 2 StoreVirtual Nodes - they are not part of the cluster other than to provide quorum or failover.

So the statement "a separate (NFSv3) file share **provided by** the two nodes of the HA cluster" is incorrect. 
This NFSv3 Share is provided by a Linux server, NetApp, or even now Windows servers can provide NFS.
The point is the Quorum Witness/FOM is an outside 3rd party who would NOT be affected by an outage of either of the 2 StoreVirtual nodes.

And note that if you did have a failure of the witness share or FOM, you lose nothing with your 2-node StoreVirtual cluster. 

Hope that clarifies things for you.

wallewek

Ah.  Thank you.  So calling this a 2-node cluster would actually be slighlty misleading, as a third computer is required,, running cluster-specific software.  Three computers are required, but only two of them need to be host servers.

A couple of thoughts:
1.  It would be interesting to see how small and "appliance-like" the third system could be.  A little box with an SSD, sitting on a shelf, for example.
2.  I've often thought that it should be possible to provide quorum-like functionality without using any computer at all.  Perhaps a cloud service, or an public DNS entry, say..  

 

HPEStorageGuy

@wallewek - might be getting into semantics on whether we call it a two-node cluster or something different but in general when we talk about a StoreVirtual cluster, the number of nodes in the cluster is the number of StoreVirtual nodes that you have. So I would call it a two-node cluster with a quorum witness. 

WRT how small can that quorum witness be - I've heard customers using a Raspberry Pi for the witness. That's pretty dang small. We think the NFS-based witness if pretty lightweight and we're not currently looking at any other types. 

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