Tool - AWS Backup Costs
Today, I was exploring some costs for a customer and discovered that the AWS Backup module was not available in the AWS Calculator. I know it was in there before, I have calculations that even include it, but it shows that they are working on it presently.
I don’t know if this indicates a change from AWS around Backup costs, but the calculation has always intrigued me. The math behind it never made 100% sense to me because it seemed to be a funny way of arriving at the estimated cost.
What I am sure of is the following:
While the underlying hardware for S3, where the backups are stored, might have block-level replication saving space, that space is not transferred to us for cost savings. If the change rate per day is 5 GB, it will be calculated for the 5 GB.
For every full backup and the initial backup, you will be charged for the size of the disk at that time and the backup sizes after that backup.
I’ve built out the formula before, but I’ve never compared it to a broken-out cost of a backup for EBS, so for those of you needing an estimate, I’ve built out a simple calculator with the way AWS calculates it and what I believe is the more accurate approach. (I don’t have all the calculations in it but you will find the frameworks for the math for the formulas that you can use to make your own)
As we can see above, two costs are in the red box: the top represents the broken-out cost, and the bottom is what I would have gotten when using the AWS Calculator Backup module.
Historically, the formula I saw them use in the module was:
Disk Size * ( (Annual Growth Rate/12) + ((Daily Change Rate+1)^30)-1) )
This is represented by the Average Monthly Change calculation in the image of 418.8305, which they would then utilize for the subsequent calculations. As we can see, there is a difference with the module coming out at $1.29, which is more expensive than the one where I’ve broken it out.
While $1.29 doesn’t seem like a lot, what if we had 1000 workloads with the same issue? $1290 per month x 12 months = $15,480, which for some orgs isn’t a lot, but this is only a daily backup with a retention of seven days. This can get out of control pretty quickly when you start calculating it for Weekly, Monthly, and Yearly backup retentions.
For estimating, I might not be too worried. Still, if I’m trying to forecast for budgetary reasons, I would definitely need to ensure that this number is as tight as possible because it would help with decisions around backup solutions and budget requests.
We will have to wait and see what AWS does because either they recognized this and put it through Q&A to resolve it, or we will see a change in AWS around backup pricing.