Local storage is divided into tiers. Within these tiers, data is handled in different ways depending on how frequently the data is being accessed. As the data moves down to less active tiers, it is compressed and deduplicated. Due to the compression and deduplication ratios, the same set of data will take up a different amount of space depending on the tier that it is stored in. Tiering is done at the block level rather than by file, so individual files can have their data split across multiple tiers.
When a storage tier is nearly full, the appliance begins moving data to the next tier by design. When the last local tier is becoming full, meaning that local storage on the appliance is full, the appliance will automatically begin to archive less active data to the cloud.
You can see the current usage of the SSD Linear, SSD Deduplication, and HDD tiers in the WebUI by navigating to Report -> Capacity -> Local Capacity Usage. Adding the numbers in these reports will provide an estimate of how much space is used on the local drives. However, it is important to note that some of the space in each tier is reserved for volume metadata.
Please see the specification data to determine the total capacity of each StorSimple product. The specifications for each model are located at http://storsimple.xyratex.com/storsimple/specifications.