You should start with the basics. Understand ZFS, pool layouts, performance tradeoffs, special VDEVs, pool free space, etc. Make appropriate choices that match your goals for your system.
BASICS
iX Systems pool layout whitepaper
https://www.truenas.com/solution-guides/#TrueNAS-PDF-zfs-storage-pool-layout/11/
Special VDEV (sVDEV) Planning, Sizing, and Considerations
This resource was originally sourced from a Suggestion thread here:
https://forums.truenas.com/t/suggestion-better-svdev-planning-oversight-tools-in-gui/394/
Why consider a sVDEV?
A sVDEV has the potential to significantly speed up the operations of a spinning hard-drive (HDD) pool. One of it’s functions is to hold metadata (ZFS produces a lot of it). The other function is to make small blocks available for file storage. These small blocks are supposed to store data that is smaller than a cut…