Stretched Server Clustering is Not a DR Plan!

Written by Joshua Stenhouse, Zerto’s UK-based Solutions Engineer.

A stretched server cluster / stretched SAN is a fantastic solution for protecting against the failure of a single SAN. However in the modern day data center with redundant power supplies, generators, controllers and networking a single SAN failure is a most unlikely event. Given the high cost of implementation, requiring high speed low latency links, matched infrastructures, same vendor storage and the relative proximity of the data centers; stretched SAN clusters are not going to deliver the BC/DR the business expects.

There are four key reasons that stretched server clustering is not right for Disaster Recovery:

1. Stretched SAN clusters are not going to protect you from logical failures.

A key issue with stretched SAN clusters is the most likely of DR events; a logical failure. This can be caused by a system wide virus, human error or multiple database corruption. With a stretched SAN cluster a logical failure is immediately written to both sites and offers no quick Recovery Time Objective (RTO) from such DR events.

Zerto protects against logical failures with the ability to failover a single application or whole site to a previous point in time within minutes, protecting against the most likely of DR events.

2. Stretched server clustering can’t protect you from site link failures.

The second most likely DR event can be a site link failure. With stretched SAN solutions this can lead to a split brain scenario with different VMs if different sites and an end result of no business continuity.

With Zerto; failover initiated manually and then Zerto automatically handles the entire failover process. This ensures that performing a DR failover is a business decision and not at automatic incorrect reaction to a link failure.

3. Close proximity of stretched clusters won’t protect you from environmental disasters.

Regional power cuts, flooding, earthquake or hurricane are all events that are commonly blamed for data center outages and failures. Due to the close proximity that stretched SAN cluster data centers need to have to ensure adequate performance, they are not likely to protect against these events.

As Zerto can replicate over much longer distances with an RPO of seconds, this is a better guaranteed line of defense against such environmental DR events.

4. Finally, stretched server clustering is high cost.

Bandwidth & storage are not cheap in a stretched cluster scenario.

Zerto is storage agnostic and replicates over existing IP networks. Existing storage can re-used in the DR site irrespective of model, connectivity or vendor, even local disks can be used. Replication can be over links as small as 5mbs with no sensitivity to latency. Couple all the above together with built in bandwidth compression and the cost of implementing Zerto is significantly lower than a stretched SAN solution.

What if you have already implemented a stretched SAN solution?

Zerto can be a great compliment to an existing environment by offering replication to a true offsite location using an IP link with no maximum latency requirement, without any performance or operational impact on the existing stretched SAN solution.

If the data and applications are of significant value to warrant the size of the existing investment required for a stretched SAN cluster, then the added layer of protection offered by Zerto Virtual Replication’s offsite replication is a no brainer.

In an enterprise infrastructure with sufficient resources the ideal solution is a stretched SAN cluster plus Zerto replicating to a different part of the world. If I had to choose between a stretched SAN solution or Zerto, I’d select the solution protecting against the most common DR events: Zerto.

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