Forced flushing creates space for new I/Os, though they dramatically affect overall performance. When forced flushing takes place, write I/Os to cache are halted while pages are being flushed. Forced flushing will stop when the percentage of dirty pages reaches the low watermark. Forced flushing will occasionally occur even in a well-designed system. Forced flushing starts when the cache is 100% full of dirty pages.  VNX OE for Block tries to keep the number of dirty pages in cache between the High and Low watermarks. The low watermark sets a minimum amount of write data to maintain in cache. It is also the point at which the VNX stops high water and forced flushing. High water flushing starts when the high watermark is exceeded. In a bursty workload environment, lowering both watermarks will increase the cache’s number of free pages beyond the High Watermark, allowing the system to absorb bursts of write requests without forced flushing. If you notice that forced flushing is occurring for most of your LUNs, decrease the watermarks by 10%. As a rule of thumb, the two watermarks should be kept about 20 percentage points apart.
How the Forced flushing occurs? What I meant is, will it happens when the percent dirty pages reaches above the HWM level or at 100% of Write Cache?  If it occurs at 100% of Write Cache then what is the benefit of changing Water Mark Levels and reduce number of forced flushing? 
How the Forced flushing occurs? What I meant is, will it happens when the 
percent dirty pages reaches above the HWM level or at 100% of Write Cache?