Buurst's best practice for SoftNAS strongly recommends the creation and use of a ZFS Intent Log, or ZIL to minimize data loss while maximizing performance.
The ZIL is referred to as the Write Log within the SoftNAS UI. The ZIL/Write Log absorbs bursts or steady streams of incoming, synchronous write operations by rapidly recording the writes in high-speed, mirrored SSD storage, then returning quickly to the caller. This makes synchronous writes operate at maximum speed. Later, when there is less I/O overhead, the writes which are cached in the ZIL are written to long-term pool storage. The ZIL is flushed every 5 seconds. For this reason, ZIL storage requirements are minimal.
Creating a write log is a simple task, outlined here.