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With a setup with two disk is there a way to migrate from software RAID1 to ZFS mirror ?

This kind of procedure would be inspired by the migration that is possible from a disk stored on a single drive to software RAID1 : setup the software RAID in degraded mode with a missing drive, copy the data, add the first drive to the array.

zpool create -f -m /data data mirror /dev/sdb 
invalid vdev specification: mirror requires at least 2 devices

Does anyone know a way of doing this ? With a fake device that writes to null ? Can ZFS change a device from "normal" to "mirror" ?

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You would create a single disk device, not a mirror. Then attach a second disk to the first disk; ZFS will automatically know it has to create a Mirror VDEV for you.

zpool create -m /data data /dev/sdb
tar cf - /old/path | tar xfC - /new/path
zpool attach data /dev/sdb /dev/sdc

insert standard warning about only having a single disk with valid data during this process and intentionally stressing the disks by copy massive amounts of data - backup, backup, backups

Side note: never, never, never use the -f argument unless some ZFS Demigod Guru instructs you to do so. I guarantee 99% of the time you don't want to force it and the other 1% of the time you'll end up destroying something.

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