Thank you for your reply, @Mobius. That does make sense. To your final note, hopefully by stable, if we manually create these vdevs in TrueNAS, they can play nicely with HexOS.
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I agree: I believe folks recommend the special vdev to have the same redundancy as the data vdevs; if it were to be added, hopefully that recommendation could be prominent. I've heard mixed things about performance: for many-file directories, it can feel snappier, which is what I hope.
SLOG: ah, I had another angle. My main interest are a couple of wired Macs on our network as Macs write to SMB synchronously (straight to HDDs), without fast RAM-first writes like Windows machines. But with an SLOG, Macs write to this faster SLOG vdev, allowing the transfer to complete faster. So with an SLOG, Macs have virtually identical write performance as Windows. But without the SLOG, Macs write a little slower.
We definitely use a UPS; it's a solid investment for data integrity as you get PLP for the whole server! A great recommendation.