OTV and Vplex: Plumbing for Disaster Avoidance

High availability, disaster recovery, business continuity, etc. are all key concerns of any data center design. They all describe separate components of the big concept: ‘When something does go wrong how do I keep doing business.’

Very public real world disasters have taught us as an industry valuable lessons in what real business continuity requires. The Oklahoma City bombing can be at least partially attributed to the concepts of off-site archives and Disaster Recovery (DR.) Prior to that having only local or off-site tape archives was commonly acceptable, data gets lost I get the tape and restore it. That worked well until we saw what happens when you have all the data and no data center to restore to.

September 11th, 2001 taught us another lesson about distance. There were companies with primary data centers in one tower and the DR data center in the other. While that may seem laughable now it wasn’t unreasonable then. There were latency and locality gains from the setup, and the idea that both world class engineering marvels could come down was far-fetched.

With lessons learned we’re now all experts in the needs of DR, right up until the next unthinkable happens ;-). Sarcasm aside we now have a better set of recommended practices for DR solutions to provide Business Continuity (BC.). It’s commonly acceptable that the minimum distance between sites be 50KM away. 50KM will protect from an explosion, a power outage, and several other events, but it probably won’t protect from a major natural disaster such as earthquake or hurricane. If those are concerns the distance increases, and you may end up with more than two data centers.

There are obviously significant costs involved in running a DR data center. Due to these costs the concept of running a ‘dark’ standby data center has gone away. If we pay for: compute, storage, and network we want to be utilizing it. Running Test/Dev systems or other non-frontline mission critical applications is one option, but ideally both data centers could be used in an active fashion for production workloads with the ability to failover for disaster recovery or avoidance.

While solutions for this exist within the high end Unix platforms and mainframes it has been a tough cookie to crack in the x86/x64 commodity server system market. The reason for this is that we’ve designed our commodity server environments as individual application silos directly tied to the operating system and underlying hardware. This makes it extremely complex to decouple and allow the application itself to live resident in two physical locations, or at least migrate non-disruptively between the two. 

In steps VMware and server virtualization.  With VMware’s ability to decouple the operating system and application from the hardware it resides on.  With the direct hardware tie removed, applications running in operating systems on virtual hardware can be migrated live (without disruption) between physical servers, this is known as vMotion.  This application mobility puts us one step closer to active/active datacenters from a Disaster Avoidance (DA) perspective, but doesn’t come without some catches: bandwidth, latency, Layer 2 adjacency, and shared storage.

The first two challenges can be addressed between data centers using two tools: distance and money.  You can always spend more money to buy more WAN/MAN bandwidth, but you can’t beat physics, so latency is dependent on the speed of light and therefore distance.  Even with those two problems solved there has traditionally been no good way to solve the Layer 2 adjacency problem.  By Layer 2 adjacency I’m talking about same VLAN/Broadcast domain, i.e. MAC based forwarding.  Solutions have existed and still exist to provide this adjacency across MAN and WAN boundaries (EoMPLS and VPLS) but they are typically complex and difficult to manage with scale.  Additionally these protocols tend to be cumbersome due to L2 flooding behaviors.

Up next is Cisco with Overlay Transport VLANs (OTV.)  OTV is a Layer 2 extension technology that utilizes MAC routing to extend Layer 2 boundaries between physically separate data centers.  OTV offers both simplicity and efficiency in an L2 extension technology by pushing this routing to Layer 2 and negating the flooding behavior of unknown unicast.  With OTV in place a VLAN can safely span a MAN or WAN boundary providing Layer 2 adjacency to hosts in separate data centers.  This leaves us with one last problem to solve.

The last step in putting the plumbing together for Long Distance vMotion is shared storage.  In order for the magic of vMotion to work, both the server the Virtual Machine (VM) is currently running on, and the server the VM will be moved to must see the same disk.  Regardless of protocol or disk type both servers need to see the files that comprise a VM.  This can be accomplished in many ways dependent on the storage protocol you’re using, but traditionally what you end up with is one of the following two scenarios:image

In the diagram above we see that both servers can access the same disk, but that the server in DC 2, must access the disk across the MAN or WAN boundary, increasing latency and decreasing performance.  The second option is:


In the next diagram shown above we see storage replication at work.  At first glance it looks like this would solve our problem, as the data would be available in both data centers, however this is not the case.  With existing replication technologies the data is only active or primary in one location, meaning it can only be read from and written to on a single array.  The replicated copy is available only for failover scenarios.  This is depicted by the P in the diagram.  While each controller/array may own active disk as shown, it’s only accessible on a single side at a single time, that is until Vplex.

EMC’s Vplex provides the ability to have active/active read/write copies of the same data in two places at the same time.  This solves our problem of having to cross the MAN/WAN boundary for every disk operation.  Using Vplex the virtual machine data can be accessed locally within each data center.


Putting both pieces together we have the infrastructure necessary to perform a Long Distance vMotion as shown above.


OTV and Vplex provide an excellent and unique infrastructure for enabling long-distance vMotion.  They are the best available ‘plumbing’ for use with VMware for disaster avoidance.  I use the term plumbing because they are just part of the picture, the pipes.  Many other factors come into play such as rerouting incoming traffic, backup, and disaster recovery.  When properly designed and implemented for the correct use cases OTV and Vplex provide a powerful tool for increasing the productivity of Active/Active data center designs.

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Disaster Recovery and the Cloud

It goes without saying that modern business relies on information technology.  As a result, it is essential that operations personnel consider the business impact of outages and plan accordingly.  As an illustration, Virgin Blue recently experienced a twenty-hour outage in its reservation system that resulted in losses of up to $20 million dollars.  The cloud provides both considerable opportunities and significant challenges relating to disaster recovery.

In general, organizations must currently build multiple levels of redundancy into their systems to reach high-availability targets and to protect themselves from catastrophic outages during a natural or man-made disaster.  A disaster recovery strategy requires that data and critical application infrastructure be duplicated at a separate location, away from the primary datacenter.  Cutting over to a disaster recovery site is usually not instantaneous and redundancy is often lost during the contingency operating plan.  For this reason, site-local redundancy mechanisms – such as high availability network systems, failover for portions of the application stack, and SAN-level redundancy are also required to achieve availability goals.  Public clouds often further complicate disaster recovery planning, as the organization’s critical systems may now be spread across their own infrastructure and a multitude of outside vendors, each with their own data model and recovery practices.

Business requirements and application criticality should guide the approach chosen for business continuity.  Consider the concepts of RPO (Recovery Point Objective) and RTO (Recovery Time Objective). The RPO of a system is the specified amount of data that may be lost in the event of a failure, while the RTO of a system is the amount of time that it will take to bring the system back online after a failure.  In general, site-local mechanisms will provide near-instantaneous RPO and RTO, while disaster recovery systems often will have an RPO of several hours or days of information, and an RTO measured in tens of minutes. Through increasingly sophisticated (and costly) infrastructures, these times can be reduced but not entirely eliminated.

Timeline illustrating concepts of RPO and RTO

Illustration of RTO and RPO in a backup system

Dedicated redundancy infrastructure, both site-local and for disaster recovery purposes, must be regularly tested.  Additionally, it is essential to ensure that the disaster recovery environment is compatible with the existing infrastructure and capable of running the critical application.  This is an area where change management procedures are important, to ensure that critical changes to the production infrastructure are made in the standby environment as well.  Otherwise, the standby environment may not be able to correctly run the application when the disaster recovery plan is activated.

The primary factor that determines RTO and RPO is the approach used to move data to the contingency site.  The easiest and lowest cost approach is tape backup.  In this case, the RPO is the time between successive backups moved off-site (perhaps a week or more) and the RTO is the amount of time necessary to retrieve the backups, restore the backups, and activate the contingency site.  This may be a significant amount of time, especially if personnel are not readily available during the disaster scenario.  Alternatively, a hot contingency site may be maintained, and database log-shipping or volume snapshotting/replication can be used to send business data to the secondary site.  These systems are costly, but readily attain an RTO of under an hour, and an RPO of perhaps one day.  With substantial investment and complexity, RPO can even be reduced to the range of minutes.  However, organizations have often been surprised to find that the infrastructure doesn’t work when it is called upon, often because of the complexity of the infrastructure and the difficulties involved in testing a standby site.

When procuring IaaS (Infrastructure as a Service) or SaaS (Software as a Service), it is essential for the organization to perform due diligence regarding what disaster recovery mechanisms the service vendor uses. The stakes are too high to trust service level agreements alone (in the case of a catastrophic failure during a disaster, will the vendor be solvent and will the compensation received be sufficient to compensate for business losses?).

Disaster Recovery as a Service, or DRaaS, is an emerging category for organizations that wish to control their own infrastructure but not maintain the disaster recovery systems themselves.  With a DRaaS offering, an IT organization does not directly build a contingency site, but instead relies on a vendor to do so on a dedicated or utility computing infrastructure.  The cloud’s advantages in elasticity and cost-reduction are significant benefits in a disaster recovery scenario, and service offerings allow organizations to outsource portions of contingency planning to vendors with expertise in the area.  However, many of the complexities remain and it is essential to perform the due diligence to ensure that the contingency plan will work and provide a sufficient level of service if called upon.

Finally, there are emerging technologies that combine site-local redundancy and disaster recovery into a unified system.  For example, distributed synchronous multi-master databases allow an application to be spread across multiple locations, including cloud availability zones, with the application active and processing transactions in all of them.  A specified portion of the system can be lost without any downtime or recovery effort.  These emerging systems offer the prospect of dramatically reducing costs and minimizing the risk of contingency sites not functioning properly.

About the Author

Michael Lyle (@MPLyle) is CTO and co-founder of Translattice, and is responsible for the company’s strategic technical direction.  He is a recognized leader in developing new technologies and has extensive experience in datacenter operations and distributed systems.

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