Hello Pille66,
"seriously ?"
Unfortunately yes, sorry to see that you only found out about it at this stage.
Using:
BaseConsumption +
(NumDiskGroups * (DiskGroupBaseConsumption + (SSDMemOverheadPerGB * SSDSize))) +
(NumCapacityDisks * CapacityDiskBaseConsumption)
In your case this adds up to approximately the consumption you have observed:
5426 MB + (1 * (636 MB + (14MB * 256))) + (1 * 70 MB)
5426 MB + (1 * (636 MB + (3584))) + (70)
=
9,716 MB
For 512GB Cache-tier:
5426 MB + (1 * (636 MB + (14MB * 512))) + (1 * 70 MB)
5426 MB + (1 * (636 MB + (7168))) + (70)
=
13,300 MB
You would likely do okay with a 256GB cache-tier SSD for a 4TB All Flash disk-group.
Bob