RAID-Zx with just 1 data disk

General Discussion
1

During another discussion about RAID-Zx expansion, it was mentioned that it was possible to create a RAID-Zx vDev with just 1 data disk;

RAID-Z1 - 2 disks minimum
RAID-Z2 - 3 disks minimum
RAID-Z3 - 4 disks minimum

Looks like Solaris 11.4 has that already. Makes me wonder if that was possible all along and was just assumed it would be silly. (Silly without RAID-Zx expansion!)

https://docs.oracle.com/en/operating-systems/solaris/oracle-solaris/11.4/manage-zfs/raid-z-storage-pool-configuration.html

You need at least two disks for a single-parity RAID-Z configuration and at least three disks for a double-parity RAID-Z configuration, and so on.

So, was it a conspiracy to hide that fact?
Or just implied silliness?
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2

Okay, now it appears that the single data disk “feature” for RAID-Zx is NOT tied to RAID-Zx expansion.

My current desktop does not have the RAID-Zx expansion feature, which is in OpenZFS master, (>2.2.5). I’m using 2.2.3 and I was allowed to create a 2 disk RAID-Z1 pool. Plus, was able to offline one of the drives and yet still have full access to the pool;

root@me:~# zpool status newpool
  pool: newpool
 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: scrub repaired 0B in 00:00:08 with 0 errors on Fri Aug 23 17:52:44 2024
config:

	NAME                             STATE     READ WRITE CKSUM
	newpool                          DEGRADED     0     0     0
	  raidz1-0                       DEGRADED     0     0     0
	    /disk1.sparse                ONLINE       0     0     0
	    /disk2.sparse                OFFLINE      0     0     0

errors: No known data errors

Next, I re-added the sparse file I had OFFLINE and removed earlier. It re-silvered just fine; 
root@me:~# zpool status newpool
  pool: newpool
 state: ONLINE
  scan: resilvered 2.33G in 00:00:10 with 0 errors on Fri Aug 23 17:57:02 2024
config:

	NAME                             STATE     READ WRITE CKSUM
	newpool                          ONLINE       0     0     0
	  raidz1-0                       ONLINE       0     0     0
	    /disk1.sparse                ONLINE       0     0     0
	    /disk2.sparse                ONLINE       0     0     0

errors: No known data errors

So, I guess single data disk RAID-Zx were always an option.
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3

Thanks for the test… yes it was feasible, but not recommended. It was blocked in the web UI.

Mirrors were faster and more reliable.

It’s only with RAID-Z expansion is there a reason for 2 drive Z1

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