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Is there any calculation for figuring out the corralation between physical (disk) storage and the amount of usable storage available to OpenStack?
I know it's not 1:1 as OpenStack duplicates data but does that make it 1:2?
As an example, in our planned infrastructure we will have 4 OpenStack servers each with 12x2TB hard drives (24TB per server so in total 96TB storage, the servers will also have 40GB SSDs for the OS) how much storage in OpenStack does that give me?

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up vote 4 down vote accepted

Since you are looking for storage numbers, I'm assuming you are referring to swift, openstack's object storage system.

Swift by default stores 3 replicas of the data. This means that total storage / 3 is your billable or usable storage. The number of replicas is configurable on a cluster level.

However, you can get more detailed. I'd recommend you keep your drives at less than, say, 80% capacity to allow for sudden bursts of storage needs and to give you time to acquire new hardware and grow your cluster. Also, keep in mind that every 2TB the manufacturer advertises is only 1.82TB. Then you have some overhead for formatting.

The above gives you "total raw storage" * 0.90949470177292824 * .8 / "replica count" = "billable storage".

Plugging in your numbers, we get 96TB * 0.90 * .8 / 3 = 23.28TB billable.

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