Traditional frame-based design
Traditional “modular” network storage system designs are frame-based. You may buy a small, medium, or large system; each frame includes controllers and shelves of disks. You can expand the system by adding disk shelves within the frame’s size limits, with each disk shelf sharing the same pair of controllers. Having a single frame provides a single point of management and a consistent feature set within the frame. With this design, before purchasing a storage system you evaluate how much capacity you need and buy a frame that fits those requirements; typically, the larger the frame, the more it costs. You can buy a larger frame than you need and over-provision for the future; however, this requires a larger capital outlay and ties up capital in unused assets. Instead you can buy a frame that initially fits only your current requirements, but this often results in future problems: expensive upgrades into larger frames, or a more challenging administrative environment with multiple frames to manage and balance.