Storage calculations

Considering the previous example, you need to plan for an initial storage capacity per server that will serve 25 VMs each.

A simple calculation, assuming 100 GB ephemeral storage per VM, will require a space of 25*100 = 2.5 TB of local storage on each compute node.

You can assign 250 GB of persistent storage per VM to have 25*250 = 5 TB of persistent storage per compute node.

Most probably, you have an idea about the replication of object storage in OpenStack, which implies the usage of three times the required space for replication.

In other words, if you are planning for X TB for object storage, your storage requirement will be 3X.

Other considerations, such as the best storage performance using SSD, can be useful for a better throughput where you can invest more boxes to get an increased IOPS.

For example, working with SSD with 20K IOPS installed in a server with eight slot drives will bring you:

(20K * 8) / 25 = 6.4 K Read IOPS and 3.2K Write IOPS

That is not bad for a production starter!