To limit the amount of RAM allocated to the ZFS ARC cache to 8 GB on TrueNAS, you can adjust a system configuration setting. Here’s how to do it:

1. **Access the TrueNAS Interface**: Log in to the TrueNAS web interface.

2. **Open the Shell**: Go to the top menu and click on **Shell** to open a terminal.

3. **Modify the ZFS Configuration**:
   - Enter the following command to limit the ARC to 8 GB:
     ```shell
     sysctl vfs.zfs.arc.max=8589934592
     ```
   - This command sets the maximum ARC size to 8 GB (8 GB in bytes = 8589934592).

4. **Make the Change Persistent**:
   - To ensure this setting applies on reboot, add it to the startup configuration file.
   - Use the following command to append it to `sysctl.conf`:
     ```shell
     echo "vfs.zfs.arc.max=8589934592" >> /etc/sysctl.conf
     ```

5. **Restart the System**: It's recommended to restart TrueNAS to make sure the setting takes effect.

This will effectively limit the ZFS ARC to 8 GB, allowing more RAM to be available for other processes.