yup. the problem is really with the SMR drives where they can (seemingly) hang for minutes at a time as they flush out the buffer track. ordinary spin-down isn't really a problem, as long as the drives spin up within a reasonable amount of time, ZFS won't drop the disk from the array.

ZFS is designed for HDD-based systems after all. actually it works notably kinda poorly for SSDs in general - a lot of the design+tuning decisions were made under the assumptions of HDD-level disk latency and aren't necessarily optimal when you can just go look at the SSD!

however, tons and tons of drive spin-up cycles are not good for HDDs. Aggressive idle timeout for power management was famously the problem with the WD Green series (wdidle3.exe lol). Best practice is leave the drives spinning all the time, it's better for the drives and doesn't consume all that much power overall. Or I would certainly think about, say, a 1-hour timeout at least.

https://www.truenas.com/community/threads/hacking-wd-greens-...

However, block-level striping like ZFS/BTRFS/Storage Spaces is not very good for spinning down anyway. Essentially all files will have to hit all disks, so you have to spin up the whole array. L2ARC with a SSD behind it might be able to serve a lot of these requests, but as soon as any block isn't in cache you will probably be spinning up all the disks very shortly (unless it's literally 1 block).

Unraid is better at this since it's a file-level striping - newer releases can even use ZFS as a backend but a file always lives on a single unraid volume, so with 1-disk ZFS pools underneath you will only be spinning up one disk. This can also be used with ZFS ARC/L2ARC or Unraid might have its own setup for tiering hot data on cache drives or hot-data drives.

(1-disk ZFS pools as Unraid volumes fits the consumer use-case very nicely imo, and that's going to be my advice for friends and family setting up NASs going forward. If ZFS loses any vdev from the pool the whole pool dies, so you want to add at least 2-disk mirrors if not 4-disk RAIDZ vdevs, but since Unraid works at a file-stripe level (with file mirroring) you just add extra disks and let it manage the file layout (and mirrors/balancing). Also, if you lose a disk, you only lose those files (or mirrors of files) but all the other files remain intact, you don't lose 1/8th of every file or whatever, and that's a failure mode that aligns a lot better with consumer expectations/needs and consumer-level janitoring. And you still retain all the benefits of ZFS in terms of ARC caching, file integrity, etc. It's not without flaws, in the naive case the performance will degrade to 1- or 2-disk read speeds (since 1 file is on 1 disk, with eg 1 mirror copy) and writes will probably be 1-disk speed, and a file or volume/image cannot exceed the size of a single disk and must have sufficient contiguous free space, and snapshots/versioning will consume more data than block-level versioning, etc. All the usual consequences of having 1 file backed by 1 disk will apply. But for "average" use-cases it seems pretty ideal and ZFS is an absolutely rock-stable backend for unraid to throw files into.)

anyway it's a little surprising that having a bunch of individual disks gave you problems with ZFS. I run 8x8TB shucked drives (looking to upgrade soon) in RAIDZ2 and I get basically 8x single-disk speed over 10gbe, ZFS amortizes out the performance very nicely. But there are definitely risks/downsides, and power costs, to having a ton of small drives, agreed. Definitely use raidz or mirrors for sure.