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

TrueNAS Comprehensive Solution Brief and Guides

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 HDD pool by putting small files and pool metadata into a faster special VDEV (sVDEV), which usually consists of SSDs. You can also designate specific datasets to reside on a sVDEV by setting record size in the dataset configuration page to equal…