Currently, I am using a USB SSD connected to a Fritzbox 7590 as a NAS. I have a small Lenovo PC running Proxmox with a 2TB M.2 SSD. Additionally, I have connected a 2TB USB SSD to it. Various VMs are running on the PC.
I have now set up a VM with TrueNAS, which is intended to replace the Fritzbox as a NAS. I want to use a 1TB partition of the M.2 SSD as a network drive. Using ZFS, the M.2 SSD should be automatically backed up to the USB SSD so that the data is not lost in case the M.2 SSD fails.
Short answer: No
Longer answer: Not in the least reliable and data safe.
Explanation:
ZFS works best with direct access to the storage device(s). Taking part of a M.2 SSD and passing only the partition to TrueNAS for ZFS has been shown to be unreliable at best, and data unsafe at worst.
However, in rare cases, doing such for the boot drive of TrueNAS can work okay. Partly because you can make 100% backup of the configuration. And if the boot drive becomes corrupt or otherwise unusable, simply re-install and restore the configuration backup.
Neither TrueNAS nor ZFS were designed for VM use, (except in the case of full device pass through). It has been shown repeatedly that the only data safe and reliable method for use of ZFS and TrueNAS is to pass the disk controller through to the VM. And in the SPECIFIC case of Proxmox, blacklist the disk controller so that Proxmox does not attempt to import the ZFS pool too.
TrueNAS and ZFS are not the most flexible NAS solution. Nor are they easy to trouble shoot, if the excrement hits the rotating impellers. (Aka if something really bad happens, *hit hits the fan…)
On the other hand, ZFS was designed for data integrity and online actions, like adding or removing non-data vDevs. And over the years, OpenZFS has added a ton of new features making it one of the top file systems, if not the top.
