Small and medium are pretty easy to calculate. If you have large instances, you will be paying £6.109 (prices as of May 2015) per 500GB so a 1.3TB backup would cost you £18.33 per month. A simple protection estate could be:
|Windows 7 laptop||45GB||£3.0545|
|Linux virtual machine||30GB||£3.0545|
And the cost for those would be £12.22 per month. So that takes care of the backup service; the agent, compression, encryption and bandwidth.
Next we need to calculate the cost of the storage. Microsoft have wisely brought this in line with the standard Azure Storage costs and you have the choice of locally redundant where your backup files are replicated three time within a single datacentre (e.g. Dublin) or zone redundant where they are replicated three times in one data centre and then three times in geographically paired datacentre (e.g. Dublin and Amsterdam).
We’ve put a typical price per GB in the table above. The actual figures vary with the amount of data you store and you can view current prices on the Azure Storage Prices. Determining the amount of storage is a bit of a guessing game as it depends on how much the data changes (the churn), how many restore points you want to keep and the level of compression that can be achieved. A file server with lots of Word documents will be compressed far more than a file server containing hundreds of .jpg images because the jpeg format is already compressed. Azure will only charge for the actual storage used so your estimate doesn’t need to be accurate. In our example, we might use the following factors:
1- the total storage of 375GB
2 – locally redundant storage because we only want an archiving and backup solution to replace tape-drives
3 – 20% of the data changes between backups
4 – 10% compression (this is conservative; a typical compression should be around 30-40% depending on the type of data being backed up)
5 – a backup every week
6 – retention period for the backups of 1 year (for a maximum of 52 backups stored after a year)
Our back-of-a-napkin calculation would be 375GB initial backup + 52 further backups would just be the data changes at 75GB (20% of 375GB). Total of 4.275TB, with compression at 10% this comes down to 3.8475TB.
So after 1 year (at which point we will have a rolling 52 backups retained), our monthly cost might be £53.87 (for storage at a rough £0.014 per GB) + £12.22 (for the protected instances) = £66.09
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