In a hybrid configuration, large-capacity spinning disks store the data, while DRAM and flash act as hyper-fast read and write caching.
The technologies work in conjunction with a flash-based separate write log (SLOG), which can be thought of as a write cache that keeps what’s called the ZFS-intent log (ZIL), used to accelerate writes. On the read side, flash can be used as a level two adaptive replacement (read) cache (L2ARC) to keep the hottest data sets on the faster flash media. Workloads with synchronous writes such as NFS and databases consistently benefit from SLOG devices, while workloads with frequently-accessed data may benefit from an L2ARC device. The reason that an L2ARC device is not always the best choice is because the level one ARC in RAM will always provide a faster cache; also, some RAM will be used by the L2ARC table.
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