Your job as an architect is to ask the right questions. When it comes to storage, make sure the growth rate or change rate is taken into account.
Consider that data growth happens no matter what, and thinking ahead avoids complicated situations where there is not enough space, so leaving some margin for future utilization is a good practice. Allowing for 10% or more space should be a good starting point, so if 50 TB spaces are requested then add 5 TB more space to the solution.
Go for the most cost-effective route. While GlusterFS allows for seamless expansion, try to avoid using this feature as an easy solution and make sure that the right size is defined from the start and a buffer is considered for future growth.