Increasing number of physical disks without impacting the pool?

[Ragnar_the_Red]

I have the following scenario:
* I have been running a TrueNAS on my custom-built SoC-based NAS for years and I am happy
* my setup is a RAIDZ1 (aka a RAID5) using 3 similarly sized disks
* in the past, I have successfully upgraded from 3x3TB disks to 3x6TB disks by replacing one disk at a time and resilvering the pool
* now, it is time to upgrade again, I want to go to 12TB disks
* at time of writing, I have already upgraded 2 of the 3 6TB disks with 12TB ones, which means my pool is currently running on 2x12TB and 1x6TB and thus the upgrade is not yet complete...
* here is the question: My NAS has 4 racks, of which I have been using only 3 thus far. I would like to put the currently empty one to use. Precisely, I would like to use 2 of the old 6TB disks, combine them in some sort of multi-drive volume to turn them into the missing third 12TB disk and then add that to my pool.

I am reasonably sure that this can be done, virtual volumes and all. However, I failed to find something in the TrueNAS manual. Drive replacement always only talks about physical drives. Can someone help me how to achieve the above?

 

[Saoshen]

the short answer is you can't.
the more correct answer is you really shouldn't.
the longer answer, some one else can explain if they wish.

 

[danb35]

I am reasonably sure that this can be done

No, it really can't. With a hardware RAID controller, the two 6TB disks could be put into a RAID0 array and that used as a member disk to your pool, but that would be all kinds of a bad idea.