Cobia: Creating fusion pool where special vdev layout differs from main pool no longer possible

[kairos]

I just installed a backup machine with Cobia and would like to replicate the disc layout with a fusion pool of the main machine with:
- 8-wide HDD in zRAID-2
- mirrored NVMe as special vdev for metadata

In the process of creating the pool in the GUI, I cannot complete the setup and get notice that the second (special) vdev should also be zRAID-2.

In bluefin, I could obtain this config via GUI (possibly by applying a 'force' setting).

Is this different behavior really intentional? Instead of reverting to the command line to add the special vdev, I would rather set this up in a controlled way in the GUI. Via command line one has to take care to use the proper gptid instead of simple device names (this could easily go wrong and cause trouble down the road).

I think, this setup is quite reasonable from a risk / performance perspective, so why should it be so heavily discouraged?

 

[Arwen]

In general, the special vDev should have similar redundancy as the main data vDevs. In your case, with RAID-Z2, that's 2 disks of redundancy. So a 2 way Mirror does not have enough redundancy. But, a 3 way Mirror would, (1 data disk, plus 2 disks of redundancy).

However, their used to be an advanced setting of some sort, that would allow different things, like this mis-matching.

It is possible that the Cobia GUI has changed enough that they had not considered this specific option. I am unable to test, but perhaps someone else can test and replicate your issue. Or find the override to allow 2 way Mirror.

If it is a real issue that the GUI does not support Mirrored Special vDev(s) on a RAID-Zx pool, then a bug report would be appropriate.

 

[ABain]

@kairos, this looks like an issue in the new UI, agree with @Arwen on this, as I can fully reproduce I'll raise a ticket now and reply here with the ticket number.

 

[ABain]

Ticket: https://ixsystems.atlassian.net/browse/NAS-125054

 

[winnielinnie]

If it is a real issue that the GUI does not support Mirrored Special vDev(s) on a RAID-Zx pool, then a bug report would be appropriate.

Being that special vDevs are usually added to increase performance, you'd assume that mirrors would be the most commonly used, regardless of the data vDev(s) layout.