Noobie error: meant to setup Hot Spare but expanded the pool by mistake

[archiboy]

I’ve got an 8x4TB raidZ2 system, running version 25.04.1. I’ve now expanded it to 9x4TB inadvertently. I understand I already have a 2 drive buffer in place. I’ve been having delayed errors on the drives thatI have been to hunting down. After changing the MB and SAS card the system is far more stable today than it was a few months back. But I’d still like the Hot Spare as belt and suspenders till I’m confident in the system. Can I remove the 9th drive after it finishes attaching it to the pool? Then re-silver based on the 8 drives? Or do I need to get into some more advanced tools to re-educate the system that it’s an 8 drive array and not 9?

[Theo]

I dont think there is any magic to undo this beyond backing it all up, destroying the pool and rebuilding correctly, then reloading the data. If you have room on the HBA, I would leave it at 9 drives and add a hot spare.

[archiboy]

Thanks for the quick response. I was hoping for a way to ‘contract’ the pool… sounds like it needs to be done right.

[Fleshmauler]

It is too late now, but this might be relevant in the future:

ZFS Pool "Checkpoints": They work just like seatbelts! (Not really)

What is a ZFS Pool “Checkpoint”? [murphys-law] [murphys-law-with-a-net] If you do destructive or experimental actions against files and folders within a filesystem, you can always resort to rolling back to a dataset’s snapshot. Even if you don’t wish to do a full rollback (which will ultimately lose any new data after the point-in-time of the snapshot), you can still retrieve old or deleted files from the read-only filesystem (i.e, "snapshot) on an individual basis. But what safeguard exi…

Having a checkpoint set up (if I’m not mistaken) is the only way I can think of reverting a pool expansion without starting over from scratch.

[archiboy]

Thx for the link. Looks like a valuable ‘seatbelt’ solution for future use. Reading through the comments it appears Checkpoints are not compatible with a pool that uses hot swap spare drives. That said a checkpoint could have saved me in this instance since I hadn’t added the hot swap device prior to executing the expansion command.