Buying storage is an ongoing process that may be periodic in some environments and seemingly continuous in others. There are several primary reasons given for storage purchases:
A common decision point in purchasing storage for all of these reasons is how much capacity to buy. There may be different types or classes of storage that segment the purchase, but the question of how much remains. Finding the answer to this is more complicated than it seems. It starts with evaluating the requirements.
Read rest of article: http://itknowledgeexchange.techtarget.com/storage-soup/estimating-required-storage-capacity-proves-tricky/