I'm in the process of ripping a bunch of my Blu-rays and I've run into an issue where Unraid is bypassing my cache and writing files directly to the array when the cache reaches about half-full. As long as the cache remains at that capacity, ALL new file transfers are bypassing the cache and being written directly to the array.
My cache drives are WD Blue 2TB SSDs, and I've been using them for over a year with no issues. I've been able to write nearly the full 2TB to the cache drives in the past (the last time I recall doing this was likely on 6.7.x).
I'm including a screenshot of the most recent example of this happening. When I took the screenshots, I was trying to transfer over 1TB of Blu-ray rips to the server. The first half of the transfer went along at a reasonable clip--about 105 MB/s consistently for about two hours. Then, all of a sudden, the transfer falls off to anywhere from 20 MB/s to 55 MB/s. I look in Unraid and see that cache drive is no longer receiving writes (at least not from this transfer) and it's going to an array disk instead.
Note in the screenshot that I've included a LibreSpeed speedtest result (LibreSpeed is hosted on Unraid in Docker). The file transfer is still happening and the speedtest was being run with that going on in the background. This speedtest thus shows spare throughput between the client and server, so it's not a networking issue.
I've also seen this issue happening in recent days trying to rip discs directly to Unraid (my usual workflow). When it reaches roughly ~960 GB of storage space used, new files ripped start getting written to an array disk. Because that can't keep up with the data from three simultaneous rips, I started ripping to my local machine, hence this batch transfer.
All these transfers are happening over SMB.
Server: Unraid 6.8.3, Xeon E5-2630v4 (10-core, 2.2GHz), SuperMicro X10DRi w/Intel i350 LAN, 32GB RAM, etc.
Client: Windows 10 v1910, Ryzen 3800X, 16 GB RAM, 2x 4TB HGST NAS in RAID1
I'm happy to provide any other relevant information if it'll help figure out what the heck is going on here.