ReFS on Storage Spaces: Is Storage Tiering working? Windows Server 2019

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I couldn't find any real information on this :-D I have Windows Server 2019 with one Storage Pool consisting of two HDDs and two SSDs with tiering and mirroring. On that there is a volume running ReFS.

How can I check if ReFS tiering is working and can the tier be set manually?

You can check it in the Performance Monitor. Add Physical disks Read and Write counters for your SSDs and HDDs. In this case, you will be able to see which Tier is being utilized by seeing which drives are doing IO. In addition, configure ReFS counter of fast and slow tiers utilization percentage. This information will give you data distribution between Tiers. In my expereience, Storage Spaces counters just do not work with ReFS, I tried it multiple times. In addition, you can read how tiered ReFS works here: https://docs.microsoft.com/en-us/windows-server/storage/refs/mirror-accelerated-parity P.S. Yeah, I know, it is written about S2D, but it works the same way in Storage Spaces it. I've tested it multiple times.

Be aware of the fact tiering with ReFS + Windows Server 2019 works in a totally different way compared to NTFS + Windows Server 2016.

2016: All writes go to the flash tier with the data being offloaded to spinning disk when flash is getting full / close to being full (~80% usable capacity occupied AFAIR).

2019: All writes go immediately to the spinning disk and data get "escalated" to the flash tier only if it's getting accessed frequently (using temperature map).

This means if you're using DiskSPD or FIO or other benchmark with WS2019 you'll be seeing disk-only performance initially. Give it some time and pinpoint smaller workload to flash to see the real performance.

P.S. It's VERY confusing! Microsoft had broken lots of apps / setups doing things like that...