I'm currently building a lab server with some cheapish hardware. 2 NVMe SSDs, bunch of 3.5 HDDs. After creating a Tiered Storage (NVMe-Mirror & HDD-parity), formating it with ReFS, everything behaves quite as it should:
- Using performance-counters I can see the fast-tier filling up, then starting to destage to the parity tier, once it hits 85%.
- I can verify new writes always hitting the fast-tier.
- Reads are issued to the fast- or slow-tier depending on data.
Only the sizing of the fast-tier seems strange:
I've used 2 * 220 GB NVMe SSds, and created a 215GB fast-tier out of it. HDDs sum up to something of 6 TB. Powershell reports this sizes as it should be:
FriendlyName TierClass MediaType ResiliencySettingName FaultDomainRedundancy Size FootprintOnPool StorageEfficiency
------------ --------- --------- --------------------- --------------------- ---- --------------- -----------------
M. Acc. Parity-NVMe-Tier Performance SSD Mirror 1 215 GB 430 GB 50,00 %
NVMe-Tier Unknown SSD Mirror 1 0 B 0 B
HDD-Tier Unknown HDD Parity 1 0 B 0 B
M. Acc. Parity-HDD-Tier Capacity HDD Parity 1 6 TB 9 TB 66,67 %
But the Issue I am Facing now: When moving on data to that tiered storage, I can see from the performance-counter that the Fast-Tier is reporting 85% Usage and starts to destage files to the slow-tier after I moved something like 40-50GB to the virtual disk.
I thought about possible reasons for this for a couple of days now, maybe somebody has an idea on that?
My current thought:
The NVMe SSDs are - as mentioned - quite cheap, so they are TLC-SSds. They can deliver quite a nice performance, as long as they are operating in pSLC Mode. However that would waste 67% of disk capacity (1 bit per cell rather than 3) - and that would kind of match my observation (33% of 220GB would be ~ 71 GB, so we are hitting 85% of total usage quite quickly)
Well, I wouldn't mind If the fast-tier is that small but on the other side doesn't have to deal with slow TLC-Performance - but why is the tier size reported as 220 GB then? And is there a way to set pSLC Mode, or is this controlled by ReFS / done by trimming etc?
I would be particulary interested, in what causes the disk to be stuck in pSLC Mode, as to my understanding, the disk should automatically switch to TLC Mode, once it runs out of available disk space. (But I also read, that MS disabled trimming with ReFS, maybe related to that?)
It seems to be intentional, or why does the ReFS-PerformanceCounter know about the actual Fast-Tier-Fill-Level, if ReFS would assume a 215GB Disk as well?
Example-Screenshot: Writing 8 GB to the fast tier, having 8GB of other data destaged, before the 8GB are deleted from the fast tier again. looks like 8GB = 10%, so ReFS is seeing the tier as ~ 80GB I'd say.
- Windows Server 2019 Standard
- Building this Lab on two nodes, can see the exact same behaviour on both nodes. (identical Hardware)
