Bandwidth – How Much is Enough?
Knowing and planning for an appropriate level of bandwidth is a key component of every DRaaS solution. In our most common DRaaS implementation, the data is moved from local repositories at the customer site over the internet or WAN to our data centers once per day, even though that data may contain multiple restore points.
Here’s a diagram showing how the data moves.
The key to an effective solution is knowing whether the remote data center is getting the data on a timely basis. We measure this daily, and we call this measurement “quality”. In other words, how current is the remote data. If it’s within the desired restore point objective (RPO) as agreed in the service level agreement (SLA), then we regard it as being “in SLA”.
It’s essential that the bandwidth between the customer site and the remote data center is sufficient to move the data that’s changed every day quickly enough that it’s in the remote data center in time to support the agreed or planned RPO. See our discussion of RPO here. As a general rule for most sites, we find that 4 to 6 hours is “quickly enough”, and this is usually scheduled overnight.
Why not use 8 or 12 hours? in more complex implementations there are other events, like multiple backup jobs, each of which use resources and must finish. So 4 to 6 hours is a conservative window of time. Some sites may be able to use longer windows – and therefore less bandwidth – to move the data. Or simply move more data per day in the longer window.
Getting to the Question
So, how much bandwidth is required to move the data in 4 to 6 hours. To answer this question, we need two pieces of data and a bit of math. First, we need to know the total storage in use. This is your disk storage in use across all servers. If you’re running VMware, one way to get this data is from your vSphere console as covered here.
Second,we need to know the daily data change rate. This is easily measured by Veeam ONE as discussed in a previous post or it can be derived from simply looking at the actual size of daily backup files.
Now for the Math
If R is required speed in Mbps (see note 1), D is data changed each day in Gb (see note 1) and T is 6 hours expressed in seconds, then our formula is: R = D / T.
To save you from number crunching, we’ve prepared this chart showing typical storage sizes and change rates and the required bandwidth result:
So at a site with a 1% daily data change rate (after compression and deduplication), which we find is typical, with storage of 5TB, the required bandwidth is 19Mb.
For a deeper dive, check out this great web based calculator at WintelGuy.com.
Note 1: WintelGuy also defines all of the relevant units of measure you may encounter.
Summarizing this Idea
- Start with your storage in use
- Find your daily data change rate
- and calculate or lookup the required bandwidth here
What if my bandwidth is not enough? There are numerous strategies we employ to deal with slower lines, larger data sizes and higher change rates. The best strategy is developed on a case-by-case basis.
Let us know if you have questions. And we welcome your thoughts and real world experiences.