Greetz
If this would be better under Linux - Software, just let me know and I'll see to moving it since most non Enterprise RAID these days is software RAID. I'm experimenting with a RockPro64 as NAS with a BTRFS RAID1 and getting horrible transfer rates (70GB took 36 hours with rsync) [Note: Yes I know rsync may have security issues with WAN but this is strictly home LAN].

I just would like to know how to benchmark transfers. I've looked at fio (massive learning curve) and iostat but the docs I've found are over a decade old which AFAIK predates BTRFS, so I'm hesitant to just jump in.

Any advice?

Just some thoughts having tried the 'raspi as NAS' thing for fun once (spoiler alert: I still run a big x86 box for 'real' use, for a reason)):

1) That awful speed could legitimately just be the limit of the device - how are you attaching the disks? Is USB involved? In every instance that I've ever tried mdadm RAID1 over USB, the performance is abysmal (70GB in 36 hours sounds about right IME). I've never bothered to troubleshoot why this happens, because its usually easier to just move over to SAS/SATA and 'do it right.'

2) Are the disks involved a bad choice for this? Most HDD makers will explicitly tell you not to use SSHDs for any RAID application because their caching will get in the way, and some (a lot?) of folks report problems with newer SMR-style hard drives (this seems to be somewhat inconsistent so I think specific configuration and drive matters, I'm also not convinced this is a 'unique' issue, and isn't just folks putting non-TLER-capable drives in parity RAID and everything is fine until it isn't, which has been a known no-no for years)

3) Is the network interface a problem? I know on a lot of those SBCs the network interface is itself a USB feature - in some cases only USB 2.0 under the hood - which can also perform badly

4) Is the device overheating or otherwise being unstable?