Today, using FreeBSD 14.3, I’m going to test booting from a drive which is a mirror of the boot drives.
This is what I’m booting from:
root@r730-04:~ # zpool list NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT zroot 923G 1.72G 921G - - 0% 0% 1.00x ONLINE - root@r730-04:~ # zpool status zroot pool: zroot state: ONLINE scan: resilvered 3.74M in 00:00:01 with 0 errors on Thu Nov 20 14:13:12 2025 config: NAME STATE READ WRITE CKSUM zroot ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 da0p3 ONLINE 0 0 0 da1p3 ONLINE 0 0 0 errors: No known data errors root@r730-04:~ #
I’m going to add this drive into the zpool:
Nov 20 15:25:29 r730-04 kernel: ada0 at ahcich1 bus 0 scbus3 target 0 lun 0 Nov 20 15:25:29 r730-04 kernel: ada0:ACS-4 ATA SATA 3.x device Nov 20 15:25:29 r730-04 kernel: ada0: Serial Number S757NS0Y700758M Nov 20 15:25:29 r730-04 kernel: ada0: 600.000MB/s transfers (SATA 3.x, UDMA6, PIO 512bytes) Nov 20 15:25:29 r730-04 kernel: ada0: Command Queueing enabled Nov 20 15:25:29 r730-04 kernel: ada0: 3815447MB (7814037168 512 byte sectors) Nov 20 15:25:29 r730-04 kernel: ada0: quirks=0x3<4K,NCQ_TRIM_BROKEN>
Preparing the drive
I first copy the partitioning from one of the boot drives:
root@r730-04:~ # gpart show ada0
gpart: No such geom: ada0.
root@r730-04:~ # gpart backup da0 | gpart restore ada0
root@r730-04:~ # gpart show ada0
=> 34 7814037101 ada0 GPT (3.6T)
34 6 - free - (3.0K)
40 532480 1 efi (260M)
532520 2008 - free - (1.0M)
534528 16777216 2 freebsd-swap (8.0G)
17311744 1936213384 3 freebsd-zfs (923G)
1953525128 5860512007 - free - (2.7T)
Then, I populated the efi partition with bootcode, as shown in Moving a zpool to larger drives #ZFS #FreeBSD.
Adding the new drive into the mirror
With this step, I am adding a third disk to the mirror. The p3 partition will be identical on all drives. Yes, one of you might object to this declaration. For what we’re doing here, the drives are identical.
root@r730-04:~ # zpool attach zroot da1p3 ada0p3 root@r730-04:~ # zpool status zroot pool: zroot state: ONLINE scan: resilvered 1.74G in 00:00:05 with 0 errors on Thu Nov 20 16:05:29 2025 config: NAME STATE READ WRITE CKSUM zroot ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 da0p3 ONLINE 0 0 0 da1p3 ONLINE 0 0 0 ada0p3 ONLINE 0 0 0 errors: No known data errors root@r730-04:~ #
Because this is so little data, it took very little time to come a proper full resilvered drive.
Removing the spare drive from the mirror
I found zpool-detach, which mentioned zpool-offline, so I tried that first, just to see how it works.
root@r730-04:~ # zpool status zroot pool: zroot state: DEGRADED status: One or more devices has been taken offline by the administrator. Sufficient replicas exist for the pool to continue functioning in a degraded state. action: Online the device using 'zpool online' or replace the device with 'zpool replace'. scan: resilvered 1.74G in 00:00:05 with 0 errors on Thu Nov 20 16:05:29 2025 config: NAME STATE READ WRITE CKSUM zroot DEGRADED 0 0 0 mirror-0 DEGRADED 0 0 0 da0p3 ONLINE 0 0 0 da1p3 ONLINE 0 0 0 ada0p3 OFFLINE 0 0 0 errors: No known data errors root@r730-04:~ # zpool detach zroot ada0p3 root@r730-04:~ # zpool status zroot pool: zroot state: ONLINE scan: resilvered 1.74G in 00:00:05 with 0 errors on Thu Nov 20 16:05:29 2025 config: NAME STATE READ WRITE CKSUM zroot ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 da0p3 ONLINE 0 0 0 da1p3 ONLINE 0 0 0 errors: No known data errors root@r730-04:~ #
Testing the backup drive
I will refer to this extra drive as the backup drive.
Next, I powered down the system, I removed the two boot drives, and I placed the backup drive into the front drive cages.
NOTE: I don’t have enough drive cages for this Dell R730. In the previous steps, the backup drive was attached to hot-swap drive cage in a PCIe slot. That’s why it had device name ada0. I could not get that drive to boot from that location. After giving up on that, I used a drive cage which had another drive in it but was not involved in this test.
With the backup drive in place, I booted. I found this:
dvl@r730-04:~ $ zpool status pool: zroot state: DEGRADED status: One or more devices could not be opened. Sufficient replicas exist for the pool to continue functioning in a degraded state. action: Attach the missing device and online it using 'zpool online'. see: https://openzfs.github.io/openzfs-docs/msg/ZFS-8000-2Q scan: resilvered 1.74G in 00:00:05 with 0 errors on Thu Nov 20 16:05:29 2025 config: NAME STATE READ WRITE CKSUM zroot DEGRADED 0 0 0 mirror-0 DEGRADED 0 0 0 da0p3 FAULTED 0 0 0 corrupted data da1p3 UNAVAIL 0 0 0 cannot open da0p3 ONLINE 0 0 0 errors: No known data errors dvl@r730-04:~ $ uptimr -sh: uptimr: not found dvl@r730-04:~ $ uptime 5:27PM up 6 mins, 1 user, load averages: 0.00, 0.00, 0.00 dvl@r730-04:~ $
Success. This is a valid method for creating a backup of your boot drives.
Remember to test them.
