This is our conditioning pass to get the units to stable state for block allocations. We run a number of fill passes over the units. Each pass takes around 42 minutes for the denser units, 21 minutes for the less dense ones. After a few passes, we hit a nice equilibrium, and performance is more deterministic, and less likely to drop as block allocations gradually fill the unit.
We run the conditioning over the complete device, one conditioning process per storage device, with multiple iterations of the passes. After 2 hours or so, and 3 passes, they are pretty stable and deterministic.
Its always fun to watch the system IO bandwidth during these passes. Each system is rocking 18-21 GB/s right now. About 90% idle on CPUs. Banging interrupts/context switches hard, but the systems are responsive.
Actually, while this is going on, we usually do our OS installation if the unit has drives for this.
I like parallelism like this …