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You don’t need zfs cache. Stay away from it. This isn’t going to help with what you want to do anyway. Just have enough RAM.
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You need to backup your stuff. Follow the 3-2-1 rule. RAID is not a backup.
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Don’t use hardware raids, there are many benefits to using software these days.
With that said, let’s dig into it. You don’t really need NVMe drives tbh. SATA is probably going to be sufficient enough here. With that said, having mirrored drives will be sufficient enough as long as you are backing up your data. This also depends on how much space you will need.
I just finished building out my backup and storage solution and ended up wanting NVMe drives for certain services that run. I just grabbed a few 1 TB drives and mirrors them. Works great and I do get better performance, even with other bottlenecks. This is then replicated to another server for backup and also to cloud backup.
You also haven’t said what hardware you are currently using or if you are using any software for the raid. Are you currently using zfs? Unraid? What hardware do you have? You might be able to use a pice slot to install multiple NVMe drives in the same slot. This requires bifurcation though.
Any reason why that board? Not 100% sure what you are trying to do, but it seems like an expensive board for a home NAS. I feel like you could get more value with other hardware. Again, you don’t need a raid controller these days. They are a pain to deal with and provide less protection when compared to software raid these days. It looks like the x16 can be split on that board to be 8/8, so if needed you can add an adapter to add 2 nvmes.
You can just get an HBA card and add a bunch of drives to that as well if you need more data ports.
I would recommend doing a bit more research on hardware and try and figure out what you need ahead of time. Something like an ASRock motherboard might better in this case. The epyc CPU is fine. But maybe get something with rdimm memory. I would just make sure it has a Management port like ipmi on the supermicro.