Interesting addition to ZFS: a L2 ARC, dubbed Readzilla. The ARC is the read-cache of ZFS. The idea behind Readzilla is to add a disk with low latency to cache random-read access to disk.

This case adds capability to ZFS to use storage devices as a level 2 ARC
(L2ARC), caching read requests that would otherwise be satisfied by disk.Random read performance can then be improved by adding fast storage devices to the L2ARC, such as short-stroked disks, solid state disks, and other media with substantially faster read latency than disk.

It´s the complementary mechanism to the seperated intent logging.