Hello everyone. I built my first two TrueNAS Scales with two mirrored discs as data pools. As everyone knows, this is offering a little bit less than 50% storage efficiency (overdraft and all). It’s all I could afford in the beginning. Time went by, I’ve learned more and I found out that by adding two more discs and going from mirror to RADz1, storage efficiency goes up to some 72% (overdraft and all)! This means that with only two more discs, I almost triple my pool storage! My hardware supports this plenty (using a secondhand Z820 workstation as a server).
The question now is: can I achieve this by using GUI Expand feature, without losing data on the two initial discs, or do I need to scrap everything and do it all over again? (that won’t be a catastrophe, as I have a backup NAS where everything is saved already).
Please explain.
Best regards and many thanks!
Unfortunately there is no non-destructive path from mirrors to RAIDZ. The mirror pool will need to be destroyed and a new pool will need to be created.
Also, RAIDZ1 is usually recommended against, especially on larger disks. In the event of a disk failure and replacement, resilver time on a RAIDZ1 pool leaves the pool at a high risk of failure.
What size are your disks?
Thank you very much for your answer. I’m using HDDs of 16 TB each.
PS: I did not consider resilver/scrubbing with this.
I personally would not use RAIDZ1 on 16TB disks unless I had a good backup, resilver time would be very risky.
Resiliency and space efficiency are best balanced by a 6-8 wide raidz2. But you need 6-8 drives to set up.
If you have exactly the total number of drives, you may hack your way from mirror to raidz, through degraded pools (here be dragons!).
What would it be good to optimize the storage performance? Getting 50% out of two discs in a mirror is not quite a dream.
PS: must consider as well that I also have a second NAS which I use to back-up the main NAS I want to expand.
As @etorix mentioned, 6-8 disks is the sweet spot for RAIDZ2. 4 disks is kind of a no mans land unfortunately.
It is hard for me to suggest this as it isn’t available yet and I haven’t tested it, but RAIDZ expansion is on the near horizon. I would potentially go for a 4w RAIDZ2 with the intention to add more disks in the future.
If you have a good, safe, reliable backup I would feel more comfortable with RAIDZ1.
Thank you all for your time and invaluable information. I’ve learned new things today. 
Also, this is a great tool to play around with and see what your end capacity will be in different configurations.
https://jro.io/capacity/
