Question about what to put on RAID and what to put on NVME

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2 HDDs (mirrored zpool), 1 SATA SSD for cache, 32 GB RAM

First read: 120 MB/s
Read while fully cached (obviously in RAM): 4.7 GB/s

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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.
You need to backup your stuff. Follow the 3-2-1 rule. RAID is not a backup.
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.

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The basic concept of the 3-2-1 backup strategy is that three copies of the data are made to be protected, the copies are stored on two different types of storage media and one copy of the data is sent offsite

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Doing that every day feels a bit impractical. I already do that every few months.

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Current hardware is an ancient fanless motherboard from 2016. RAID6 is through mdadm. Four of the drives are through a super slow PCIe 2.0 1x card.

New motherboard (just ordered) is a supermicro H13SAE-MF which has dual nvme slots and a built in raid controller for them.

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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.

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I wanted to get something with a lot of upgrade potential, and this was the cheapest option to get my foot in the door with an EPYC processor.

Also needed two PCIe slots that could do at least 8x for the hba card and Intel ARC for video streaming.

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All a matter of your risk tolerance and how often the data changes.

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Don't make the same mistake I did. Get a backup in place before using ZFS. Using ZFS and RAIDing your drives together makes them a singular failure point. If ZFS fucks up, you're done. The only way to mitigate this is having another copy in a different pool and preferably different machine. I got lucky that my corrupted ZFS pool was still readable and I could copy files off, but others have not been so lucky.

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Yeah, I wouldn't dare.

The fact that I migrated from a 3 drive to 6 drive mdadm raid without losing anything is a damn miracle.

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Oh I wasn't saying to not, I was just saying make sure you're aware of what recovery entails since a lot of raid controllers don't just write bytes to the disk and can, if you don't have spares, make recovery a pain in the ass.

I'm using MD raid for my boot SSDs and yeah, the install was a complete pain in the ass since the debian installer will let you, but it's very much in the linux sense of 'let you': you can do it, but you're figuring it out on your own.

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Where I've landed now is

A) just migrate everything over so I can continue working.
B) Migrate my mdadm to ZFS
C) Buy another NVME down the road and configure it with the onboard RAID controller to prevent any sudden system downtime.
D) Configure nightly backups of anything of import on the NVME RAID to the ZFS pool.
E) Configure nightly snapshots of the ZFS pool to another webserver on-site.
F) rsync the ZFS pool to cold storage every six months and store off-site.

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I have run photoprism straight from mdadm RAID5 on some ye olde SAS drives with only a reduction in the indexing speed (About 30K photos which took ~2 hours to index with GPU tensorflow).

That being said I'm in a similar boat doing an upgrade and I have some warnings that I have found are helpful:

Consumer grade NVMEs are not designed for tons of write ops, so they should optimally only be used in RAID 0/1/10. RAID 5/6 will literally start with a massive parity rip on the drives, and the default timer for RAID checks on Linux is 1 week. Same goes for ZFS and mdadm caching, just proceed with caution (ie 321 backups) if you go that route. Even if you end up doing RAID 5/6, make sure you get quality hardware with decent TBW, as sever grade NVMEs are often triple in TBW rating.
ZFS is a load of pain if you're running anything related to Fedora or Redhat, and the performance implications from lots and lots of testing is still arguably inconclusive on a NAS/Home lab setup. Unless you rely on the specific feature set or are making an actual hefty storage node, stock mdadm and LVM will probably fulfill your needs.
Btrfs has all the features you need but is a load of trash in performance, highly recommend XFS for file integrity features + built in data dedup, and mdadm/lvm for the rest.

I'm personally going with the NVME scheduled backups to RAID because the caching just doesn't seem worth it when I'm gonna be slamming huge media files around all day along with running VMs and other crap. For context, the 2TB NVME brand I have is only rated for 1200 TBW. That's probably more then enough for a file server, but for my homelab server it would just be caching constantly with whatever workload I'm throwing at it. Would still probably last a few years no issues, but SSD pricing has just been awful these past few years.

On a related note, Photoprism needs to upgrade to Tensorflow 2 so I don't have to compile an antiquated binary for CUDA support.

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Question about what to put on RAID and what to put on NVME