Is it bad to have one vdev with 18x 4tb ssd

[SmallBarky]

Without knowing how you use the server, we are just guessing. What are your priorities?

I can gauge your knowledge level so linking basic articles
BASICS

ZFS Primer

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

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…