Most of the time you want your ZFS working as fast as possible, but there are occasions where you want to slow down thing a little bit not to disturb the real work too much. Resilvering or scrubing of a ZFS pool is such an example. A ZFS resilvering can be very resource hungry.

Now there is a mechanism to prevent the system from experiencing such a slowdown by reducing the speed of resilvering/scrubbing in favor of more important I/O . The code enabling Opensolaris to do so found its way to the source yesterday. The mechanism is relatively easy: Whenever there was a read or a sync write to an pool without seperate log in the last 50 timer ticks, there is an additional delay of 2(resilvering) or 4(scrubbing) ticks between the scan steps.

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