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The ZFS Intent Log (ZIL) 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 

On AWS some customers use a small (5 GB) Provisioned IOPS SSD (never direct attached ephemeral SSD) to absorb massive write bursts. It does not need to be large because it only buffers a small timeframe of data.

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