Within two weeks of backups being written to Blog Storage, we observed a significant upward trend in cost associated with a Storage Account. When compared to the previous month, there was an increase of c. £270. After some further analysis, we were able to associate this increase with “bandwidth” charges. This didn’t feel right – you don’t pay anything to upload data to Azure (ingress), you only pay when downloading data from Azure (egress).
Using Azure Monitor, we profiled the ingress and egress rates for the affected Storage Account and noticed the following pattern:
Each day, c. 150GB of backups were being written to blob storage (in blue), but shortly after, the same amount was being downloaded (in red). Over this period, we calculated 4TB of data had been uploaded and then downloaded again. Based on Microsoft’s latest Bandwidth pricing, whilst the first 5GB of egress per month is free, the next 5GB – 10TB is charged at £0.065 per month. Some simple maths confirms it to be the additional £270 we observed.
Read on for three possible solutions. My preference for an on-prem solution would be to verify locally and then push to Blob Storage / S3. Backups tend to be faster that way as well, as your disk is likely faster than your network.