I currently own 2 drives, which are unassigned in TrueNAS. I want to add one more later, but I need to use that storage now. Is there a way I can create a RAID mirror now and convert it to RAID Z1 later on? Or can I create a RAID Z1 array with just 2 drives in TrueNAS.
You can create a degraded Z1 by using a sparse file on existing storage (as a disk) and them immediately removing it (the fake disk).
I don’t actually know how as I have never done it - but it is doable. Note that you will have no parity whilst running degraded.
You cannot convert a mirror to a Zn whilst keeping the data
Raidz1 supports just two devices, you can create a pool using that, probably via CLI only, then later expand it using raidz expansion.
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That I did not know
Got it no direct way to convert a mirror into RAIDZ1 without rebuilding, and using 2 drivers for RAIDZ1 isn’t really recommended. I’ll just wait until I have the third drive
A two wide raidz vdev is not recommended versus a mirror because it will perform worse, especially for reads, since the devices won’t be striped, but if the intention is to expand it soon, it would be fine, though if you can wait for the three devices, you won’t have to deal with the incorrect used/free space reporting zfs bug after a raidz expansion.