Great article describing level one and two memory caching in zfs."L2ARC is a new layer between Disk and the cache (ARC) in main memory for ZFS. It uses dedicated storage devices to hold cached data. The main role of this cache is to boost the performance of random read workloads. The intended L2ARC devices include 1K/15K RPM disks like short-stroked disks, solid state disks (SSD), and other media with substantially faster read latency than disk." Understanding ZFS & ZFS ARC/L2ARC