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.
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Agreed. I can see situations that this would protect against versus keeping them all in the mirror - but every one of them would be better served by putting that drive in another box and sending snapshots to it.
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Even without the other box and OTA replication, I don’t see the advantage to offlining a mirror vdev member and re-onlining it periodically as opposed to simply replicating onto the single drive as its own pool.
I talk a lot about how unreliable USB drives are, but this is very much a use case that begs for one, if you’ve got a small enough set of data to fit on a single drive. Plug in your USB drive, zpool import it, syncoid from your main pool onto the removable, zpool export it and done.
Although you should really also have a schedule for scrubbing your backup. That’s where we really start getting into the “better to have warm backups than cold,” because in addition to folks who won’t keep up with a manual backup routine, there are legions of folks who aren’t going to bother not only manually backing up regularly, but manually scrubbing and waiting for the results regularly.
Safer to have something you can automate and monitor, in my experience. MUCH safer.
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