Many times I end up discussing HA and DR with customer and colleagues. Hence I decided that defining this and sharing my view on it is a good wait to start this blog 🙂
Today most IT folks mix up DR and HA, due to the possibilities the industry gave with technologies like stretched clusters, synchronous storage mirroring etc. etc.
High Availability (HA) his about Local Availability, Downtime Avoidance and Disaster Avoidance. Disaster Recovery (DR) on the other hand is all about Site Availability and Recovery ,for example on how and where to recover after a disaster that has wiped out the entire datacenter. Downtime avoidance and disaster avoidance is mainly driven by Operation Level Agreements (OLA) and Service Level Agreements (SLA). Whereas disaster recovery is driven by Recovery Time Objective (RTO: how long does it take to bring the application online again) and Recovery Point Objective (RPO: how long is the time between two backups, so to say how many data am I allowed to loose)
But let’s have a deeper look into this: Enhancing local availability is used for improving the uptime of an application in failure or maintenance situations. This we can be achieved with technologies like clustering, vMotion, Livemigration for local sites. So this is HA and typical HA is site local, because with HA I do not react on complete outages of a site.
For situation where your complete site is failing, e.g. a plane crashing on your site or a power blackout, DR is used to protect your data against this. This you can’t archive with HA and stretched clusters ( a cluster across two sites), because when your data is gone on the primary site there is nothing left to migrate over to the other site. Backup strategies are also part of DR. Stretched cluster (storage, compute, etc.) do not protect you against config failures or accidentally deleted data, because the information is immediately synchronized to the remote site.
Stretched cluster (e.g. A ESXi Cluster across 2 sites) require to have L2 extension between both sites. Conclusion of this is that you have more less 1 virtual DC across 2 physical sites. This is more or less an Active/Active datacenter, with all benefits and drawbacks (this is a definitely another discussion for a future blog entry).
As conclusion on this HA and DR discussion:
Use HA for improving local downtime & disaster avoidance and DR for cross site disaster recovery and backup/restore strategies.
Stop mixing both strategies, because it will never give you a real DR solution. As somebody said “ No, DropBox is not a Backup”!