Symptoms

The following provides guidance on guides when to add a read reading log or a write written log for the Softnas pools.

Purpose

The following provides guidance on guides when to add Write Log (Intent Log/SLOG) and  Read Cache (L2ARC) to the Softnas pools.

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ZFS intent log can be called ZIL or write log but when you assign a separate disk device as an intent log then it's also called SLOG or separate intent log. Many people get confused and consider it as a write cache that pushes data to the pool. But the ZFS write log works in a different waydifferently. The intent log only gets utilized when there are synchronous write requests to the pool. Synchronous write requests only improve speed in those situations when the write to the actual storage array would take longer than writing to the ZIL, and it doesn't actually write any quicker it only returns from the synchronous write request quicker (so the application thinks stuff is written to disk quicker, but the actual write throughput is not improved on the whole). That means the sync writes are not committed to the main storage immediately; it just means they’re buffered on - disk in a way that will survive a crash if necessary.

By default both NFS and Samba writes are asynchronous. That means if you mount a Softnas NFS or CIFS share to a client the write log will not be utilized. The intent log doesn't have to be very big and it can be a few gigabytes. ZIL can be very useful for the databases where writes are synchronous.

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