Hi
We have a 6 node HCI S2D Cluster. SSD cache and HDD Capacity disks. The pool already has 10 LUNs (virtual disks) and still has 40TB of free space. All these LUNs are 3-way mirror. These LUNs are all 4TB. In the past when I created another 4TB LUN, I knew the footprint would be 3 times as high, so 12TB. I would see the pool size shrink with 12TB.
So, now I have 40TB free, so this means I have at least 3 more LUNs to go before I empty out my pool (3X12=36, so 4 TB leftover).
But something weird happens. When I create a 4TB LUN (virtual disk) I get an error saying it cannot create this new virtual disk.
My first reaction was I might have some Storage Jobs still running in the background. Checked, but there were none.
Next I thought I might have big imbalance of data-blocks across my nodes. I used Prettypool.ps1 and it looked pretty enough to me. But I decided to Optimize the pool anyway. After that finished, I retried but still the same error.
Next I created a 2TB LUN instead. This worked. Then I created another 2TB LUN. Which also worked. Hang on, since when does 2+2 no longer makes 4 ?
So I was thinking, colums, faultdomains, etc. etc. But, I never changed any of those. Faultdomain is at the default settting of Node. I never change any other settings for 3-way mirror either.
When creating the disk, you see both storage tiers and they display their individual left-over footprint sizes, making up that 40TB free poolsize. See below in yellow highlight. 13.382TB of free space left for 3-way mirror disks.
This is my first time making more then 10 LUNs (virtual disks) on a pool. Is there some limit ? OK but why can I not create a 4TB disk, while at the same time I can create 2 disks of 2TB ?
The pool used to have Dual-Parity disks, and even a Hybrid volume with Mirror-accelerated parity. Those are deleted now, and the space has been re-claimed.
I also miss a couple of months of Windows Updates (like 4), but I don't seem to remember reading any release notes with a fix for this.
Or am I missing something about the way S2D works ?
Any help is welcome.