Is there a downside for purposely keeping one disk offline from a mirror pool?

I was wondering if there is a downside for keeping a disk purposedly offlined from a mirror pool. F.e. if I have a mirror of 4 disks from which I take one offline for use as a cold backup, periodically switching the disks (offline another one and online the previously offlined disk and let it resilver). There will be read speed penalties ofc.

Do you mean that you have two drives and only one drive on line at any given time? If that’s your situation, you’ve lost any redundancy on the (sort of) RAID. If a drive malfunctions, you lose any changes since the previous swap.

If you mean that you have a mirror of three drives and swap in a cold spare from time to time, I don’t see any downside.

I regularly (every 6 months to a year) rotate a cold spare into my 5 drive RAIDZ2. I do this to distribute hours of operation on the drives. That leaves me with full redundancy at all times and helps reduce the hours of operation on remaining drives.

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I mean that I would have like 4 way mirror with 3 drives always connected. I’m talking about 8tb drives and as such I would like to have more than 1 drive amount of redundancy because it takes time to resilver in case of disk failure and I do not have a quick way of getting new ones.

4 way mirror with 3 drives always connected.

That’s not a lot different from what I do.

You need to make sure whatever monitoring you’re doing doesn’t rely on the distinction between ONLINE and DEGRADED, since I believe having an OFFLINE device results in a DEGRADED pool.

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Yes, the pool status does show as degraded but that isn’t a problem considering that this will be a manually monitored homelab pool

I don’t consider this a backup.

A backup should be in a physically separate chassis, even if it was just a single disk zpool.

Manually zpool online ing it to let the drive catch up also sounds like a hassle.