As far as I can figure, Storage Spaces will not use the fast tier (SSD) if the fast tier is below a particular percentage (~3.2%) of the total size of the virtual drive.
This is a PITA because 1TB of a fast tier with a large dual parity capacity tier still speeds up large writes nicely.
The ReFS performance counters show that what SS considers for fast tier size and fill % is adjusted based on the capacity tier, at least to some degree. As I shrunk the capacity tier the fast tier % full more closely matched the size of the fast tier.
I have scoured the interwebs and found many people with issues getting tiering working and I now think most of the problems are from the fast tier being below some threshold value that is not configurable.
I adjusted the registry key above, rebooted and re-created a VHD with the same results. The key does work as far as adjusting when the fast tier destages but it does not have any impact on this problem with where writes land.
Does anybody know if there is another registry key that can be added to control this threshold?
SSD Tier | HDD Tier | Data on VHD | ReFS reported Fast Tier Data Fill % |
1TB | 30TB | 3.76GB | 2% |
1TB | 31TB | 3.76GB | 8% |
1TB | 32TB | 3.76GB | N/A* |
1TB | 10TB | 60.0GB | 8% |
1TB | 10TB | 400.0GB | 51% |
*All writes go directly to slow tier even when drive has zero data on it.
I don't think NTFS worked even when I shrunk the capacity tier.