EBS - monthly pricing means monthly pricing

I just made a rather surprising discovery that I wanted to quickly share/jot down regarding EBS. My usecase is that I'm trying to gather up all the provisioned storage that we have in $segment so I can compare it to some other metrics reporting I have on the topic.

The metrics I have are reporting from Ceph, which is a pooled storage solution that includes replication etc etc, so that the amount of EBS you buy from AWS - say 1024GB - is not what you'll have available to use in Ceph. If you configured Ceph to run with 1 replica then your available storage from that 1024 would actually only be 512GB (tremendously simplifying here).

So, like all things, I turn to my bill to see what it has to tell me about the amount of storage that we've purchased for this $segment.

Now, I logged in to the AWS panel to verify the size of this volume to be 1024GB, so I know that's what I'm working against. My thought process is "how does 1024GB work into 33.03 GB/mo"? Typically I would think I need to multiply the GB/mo by 720 - the number of hours in a 30 day month and my goto for how many hours are in a month, more on this later - and then divide by 24 to land at the size of the volume (assuming it doesn't change). However:

(33.03 * 720) / 24 = 990 hmmm. So wait, what's 1024 / 33.03? To my surprise it's 31, which is odd, and yes there were 31 days in May but wait, what's up with June 1? Zoom out! Enhance!

Oh no. Oh no, no, no, no. Guess what 1024 / 34.13 is? If you guessed "the number of days in the month of June" you are the winner! I'm very surprised to find this out - that they daily usage the same EBS volume actually changes depending on the number of days in the month - but not as surprised as when I add the cost in...

Yes, that's right. The daily cost of your EBS volume also changes depending on the length of the month. So, to borrow a phrase, my mental model for how the world works was wrong and I must now update it. I would also like to point out again where I think Microsoft has chosen such user-friendly standards for the people who pay the bills while building out Azure. A month in Azure is not 720 hours, nor 744, it's 730, which is 24 * 365 / 12 which makes for an extremely logical and predictable experience when trying to figure out what things cost.

I'm still mid-investigation so that's all I have for now, typically if I wait until I've got it totally figured out the shock of it has worn off and I no longer feel like posting. Cheers.