When it comes to the enterprise technology stack, nothing has captured my heart &  imagination quite like enterprise storage systems.


Stephen Foskett once observed that all else is simply plumbing, and he’s right. Everything else in the stack exists merely to transport, secure, process, manipulate, organize, index or in some way serve & protect the bytes in your storage array.


But it's complex to manage, especially in small/medium enterprises where the storage spend is rare and there are no do-overs. If you're buying an array, you've got to get it right the first time, and that means you've got to figure out a way to forecast how much storage you actually need over time.


I've used Excel a few times to do just that: build a model for storage capacity. Details below!


Using Excel to model storage capacity

Open up Excel or your favorite open-sourced alternative, and input your existing storage capacity in its entirety. Let’s say you have –direct, shared, converged, or otherwise- 75 terabytes of storage in your enterprise.


Now calculate how much of that is available for use both in absolute terms (TBs!) and as a percentage of used, committed space. Of that 75TB, maybe you've got 13 terabytes left  as usable capacity. Perhaps some of that 13TB is in a shared block or NFS storage array; perhaps all that remains for your use is direct-attached storage inside production servers. If the latter, you need to find a way to reflect the difficulty you’ll have in using the free capacity you have.


Now array that snapshot of your existing storage (75TB, 13TB free, 83% Committed) in separate columns, and in a new column, populate Month 1, 2, 3 all the way out to 36 or 72. What's your storage going to look like over time?


The reality is that unless you work in one of those rare IT environments that’s figured out the complex riddle of data retention, most IT environments will see storage demand grow over time. Use this to your advantage.


Sometimes that growth is predictable, other times it’s not. Reflect both scenarios in the same column. Assume months 1-12 there will be a .3% demand for more storage from your 13TB of remaining storage (about 39 GB every month).


In month 13, imagine that some big event happens; a new product launch or merger/acquisition, etc, and demand that month is extreme: the business needs about 15% of what (by then is) about 12.7TB of your storage. If you’re in a place that only has direct-attached storage, this might be the month your juggling act becomes especially acute; what if none of your servers has 2 Terabytes of free space?


By now your storage capacity model should be taking shape. You can plug in different categories of storage (Scale up your S3 or Azure Blob storage to meet sudden spikes in demnad, for instance,) you can contemplate disposal of some of your existing legacy storage too.



Once all your model is finished, you should be able to make a nice chart to present to your CIO, a model that says you've done your homework answering the question "How much storage do we need?"