@dwma Welcome to the forums!
This is unfortunately a complex topic. TrueNAS 24.10 and 25.04 cannot be used with the built-in
ZFS over iSCSI storage type, due to (I am simplifying
a lot) TrueNAS (1) not supporting any of the iSCSI target protocols that Promox implements (none of Lio, IET, ISTGT or Comstar are supported); and (2) Proxmox needing SSH access to TrueNAS to run ZFS commands as root, which the TrueNAS developers
do not like and flatly discourage.
A Github user named TheGrandWazoo previously wrote a third party add-on storage plugin for Proxmox that made it compatible with TrueNAS, but TrueNAS updates in 24.10 broke that plugin. It also didn't work in TrueNAS 25.04 due to API changes by the TrueNAS developers.
Some volunteers explored this situation and realized that the changes to TrueNAS broke the Proxmox plugin hard enough that it can't easily be fixed. A new plugin is being developed now that (1) replicates the old functionality; and (2) is compatible with TrueNAS 24.10 and 25.04. Eventually, that new plugin won't be dependent on SSH and ZFS commands, but for now it is. The plugin is currently in alpha testing and not anywhere close to ready for production use.
I went down a rabbithole trying to figure out the state of ZFS over iSCSI and TrueNAS in 2025, and wrote up a summary of my learnings here, including links that go to statements from the PVE and TrueNAS devs about the current state of affairs:
https://discourse.practicalzfs.com/...t-api-truenas-24-10-and-25-05-compatible/2454 It's a lot to read but I'd recommend it if you have time. Understanding why all this works the way it does makes things less confusing and frustrating.
If you just want to jump into testing the alpha, non-production new plugin, check out the new repo:
https://github.com/boomshankerx/proxmox-truenas
