SLOG Devices

To improve read performance, ZFS uses system memory as an Adaptive Replacement Cache (ARC). System memory stores the most frequent and recent ZFS pool data used. Layer 2 ARC (L2ARC) is another option to extend the ARC to dedicated disks for dramatic improvement in read speeds versus slower data disks in the pool.

Another OpenZFS performance feature is the ZFS intent log (ZIL). The ZIL writes synchronous transactions to disk in special pre-allocated space so ZFS can confirm that data is on non-volatile storage, providing lower latency for synchronous writes. The data is then written to disk in a normal transaction group (TXG) which allows reusing of the ZIL space for further writes.