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?
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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.
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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!).
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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/
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